Recombineering: genetic engineering in bacteria using homologous recombination

The inactivation of tolC sensitizes Escherichia coli to perturbations in lipopolysaccharide transport

The Escherichia coli outer membrane channel TolC complexes with several inner membrane efflux pumps to export compounds across the cell envelope. All components of these complexes are essential for robust efflux activity, yet E. coli is more sensitive to antimicrobial compounds when tolC is inactivated compared to the inactivation of genes encoding the inner membrane drug efflux pumps. While investigating these susceptibility differences, we identified a distinct class of inhibitors targeting the core-lipopolysaccharide translocase, MsbA. We show that tolC null mutants are sensitized to structurally unrelated MsbA inhibitors and msbA knockdown, highlighting a synthetic-sick interaction. Phenotypic profiling revealed that tolC inactivation induced cell envelope softening and increased outer membrane permeability. Overall, this work identified a chemical probe of MsbA, revealed that tolC is associated with cell envelope mechanics and integrity, and highlighted that these findings should be considered when using tolC null mutants to study efflux deficiency.

The crystal structure of bacteriophage λ RexA provides novel insights into the DNA binding properties of Rex-like phage exclusion proteins

RexA and RexB function as an exclusion system that prevents bacteriophage T4rII mutants from growing on Escherichia coli λ phage lysogens. Recent data established that RexA is a non-specific DNA binding protein that can act independently of RexB to bias the λ bistable switch toward the lytic state, preventing conversion back to lysogeny. The molecular interactions underlying these activities are unknown, owing in part to a dearth of structural information. Here, we present the 2.05-Å crystal structure of the λ RexA dimer, which reveals a two-domain architecture with unexpected structural homology to the recombination-associated protein RdgC. Modelling suggests that our structure adopts a closed conformation and would require significant domain rearrangements to facilitate DNA binding. Mutagenesis coupled with electromobility shift assays, limited proteolysis, and double electron-electron spin resonance spectroscopy support a DNA-dependent conformational change. In vivo phenotypes of RexA mutants suggest that DNA binding is not a strict requirement for phage exclusion but may directly contribute to modulation of the bistable switch. We further demonstrate that RexA homologs from other temperate phages also dimerize and bind DNA in vitro. Collectively, these findings advance our mechanistic understanding of Rex functions and provide new evolutionary insights into different aspects of phage biology.

Proteolytic stability and aggregation in a key metabolic enzyme of bacteria

Proteins that are kinetically stable are thought to be less prone to both aggregation and proteolysis. We demonstrate that the classical lac system of Escherichia coli can be leveraged as a model system to study this relation. β-galactosidase (LacZ) plays a critical role in lactose metabolism and is an extremely stable protein that can persist in growing cells for multiple generations after expression has stopped. By attaching degradation tags to the LacZ protein, we find that LacZ can be transiently degraded during lac operon expression but once expression has stopped functional LacZ is protected from degradation. We reversibly destabilize its tetrameric assembly using α-complementation, and show that unassembled LacZ monomers and dimers can either be degraded or lead to formation of aggregates within cells, while the tetrameric state protects against proteolysis and aggregation. We show that the presence of aggregates is associated with cell death, and that these proteotoxic stress phenotypes can be alleviated by attaching an ssrA tag to LacZ monomers which leads to their degradation. We unify our findings using a biophysical model that enables the interplay of protein assembly, degradation, and aggregation to be studied quantitatively in vivo. This work may yield approaches to reversing and preventing protein-misfolding disease states, while elucidating the functions of proteolytic stability in constant and fluctuating environments.

Characterization and Engineering of the Type 3 Secretion System Needle Monomer from Salmonella Through the Construction and Screening of a Comprehensive Mutagenesis Library

Protein production strategies in bacteria are often limited due to the need for cell lysis and complicated purification schemes. To avoid these challenges, researchers have developed bacterial strains capable of secreting heterologous protein products outside the cell, but secretion titers often remain too low for commercial applicability. Improved understanding of the link between secretion system structure and its secretory abilities can help overcome the barrier to engineering higher secretion titers. Here we investigated this link with the PrgI protein, the monomer of the secretory channel of the Type 3 Secretion System (T3SS) of Salmonella enterica . Despite detailed knowledge of the PrgI needle's assembly and structure, little is known about how its structure influences its secretory capabilities. To study this, we recently constructed a comprehensive codon mutagenesis library of the PrgI protein utilizing a novel one pot recombineering approach. We then screened this library for functional T3SS assembly and secretion titer by measuring the secretion of alkaline phosphatase using a high-throughput activity assay. This allowed us to construct a first-of-its-kind secretion fitness landscape (SFL) to characterize the PrgI needle's mutability at each position as well as the mutations which lead to enhanced T3SS secretion. We discovered new design rules for building a functional T3SS as well as identified hypersecreting mutants. This work can be used to increase understanding of the T3SS's assembly and identify further targets for engineering. This work also provides a blueprint for future efforts to engineer other complex protein assemblies through the construction of fitness landscapes.

Importance

Protein secretion offers a simplified alternative method for protein purification from bacterial hosts. However, the current state-of-the-art methods for protein secretion in bacteria are still hindered by low yields relative to traditional protein purification strategies. Engineers are now seeking strategies to enhance protein secretion titers from bacterial hosts, often through genetic manipulations. In this study, we demonstrate that protein engineering strategies focused on altering the secretion apparatus can be a fruitful avenue toward this goal. Specifically, this study focuses on how changes to the PrgI needle protein from the type 3 secretion system from Salmonella enterica can impact secretion titer. We demonstrate that this complex is amenable to comprehensive mutagenesis studies and that this can yield both PrgI variants with increased secretory capabilities and insight into the normal functioning of the type 3 secretion system.

The sensor of the bacterial histidine kinase CpxA is a novel dimer of extracytoplasmic Per-ARNT-Sim domains

Histidine kinases are key bacterial sensors that recognize diverse environmental stimuli. While mechanisms of phosphorylation and phosphotransfer by cytoplasmic kinase domains are relatively well-characterized, the ways in which extracytoplasmic sensor domains regulate activation remain mysterious. The Cpx envelope stress response is a conserved Gram-negative two-component system which is controlled by the sensor kinase CpxA. We report the structure of the Escherichia coli CpxA sensor domain (CpxA-SD) as a globular Per-ARNT-Sim (PAS)-like fold highly similar to that of Vibrio parahaemolyticus CpxA as determined by X-ray crystallography. Because sensor kinase dimerization is important for signaling, we used AlphaFold2 to model CpxA-SD in the context of its connected transmembrane domains, which yielded a novel dimer of PAS domains possessing a distinct dimer organization compared to previously characterized sensor domains. Gain of function cpxA∗ alleles map to the dimer interface, and mutation of other residues in this region also leads to constitutive activation. CpxA activation can be suppressed by mutations that restore inter-monomer interactions, suggesting that inhibitory interactions between CpxA-SD monomers are the major point of control for CpxA activation and signaling. Searching through hundreds of structural homologs revealed the sensor domain of Pseudomonas aeruginosa sensor kinase PfeS as the only PAS structure in the same novel dimer orientation as CpxA, suggesting that our dimer orientation may be utilized by other extracytoplasmic PAS domains. Overall, our findings provide insight into the diversity of the organization of PAS sensory domains and how they regulate sensor kinase activation.

Employing synthetic biology to expand antibiotic discovery

Antimicrobial-resistant (AMR) bacterial pathogens are a continually growing threat as our methods for combating these infections continue to be overcome by the evolution of resistance mechanisms. Recent therapeutic methods have not staved off the concern of AMR infections, so continued research focuses on new ways of identifying small molecules to treat AMR pathogens. While chemical modification of existing antibiotics is possible, there has been rapid development of resistance by pathogens that were initially susceptible to these compounds. Synthetic biology is becoming a key strategy in trying to predict and induce novel, natural antibiotics. Advances in cloning and mutagenesis techniques applied through a synthetic biology lens can help characterize the native regulation of antibiotic biosynthetic gene clusters (BGCs) to identify potential modifications leading to more potent antibiotic activity. Additionally, many cryptic antibiotic BGCs are derived from non-ribosomal peptide synthase (NRPS) and polyketide synthase (PKS) biosynthetic pathways; complex, clustered genetic sequences that give rise to amino acid-derived natural products. Synthetic biology can be applied to modify and metabolically engineer these enzyme-based systems to promote rapid and sustainable production of natural products and their variants. This review will focus on recent advances related to synthetic biology as applied to genetic pathway characterization and identification of antibiotics from naturally occurring BGCs. Specifically, we will summarize recent efforts to characterize BGCs via general genomic mutagenesis, endogenous gene expression, and heterologous gene expression.

Chimeric systems composed of swapped Tra subunits between distantly-related F plasmids reveal striking plasticity among type IV secretion machines

Bacterial type IV secretion systems (T4SSs) are a versatile family of macromolecular translocators, collectively able to recruit diverse DNA and protein substrates and deliver them to a wide range of cell types. Presently, there is little understanding of how T4SSs recognize substrate repertoires and form productive contacts with specific target cells. Although T4SSs are composed of a number of conserved subunits and adopt certain conserved structural features, they also display considerable compositional and structural diversity. Here, we explored the structural bases underlying the functional versatility of T4SSs through systematic deletion and subunit swapping between two conjugation systems encoded by the distantly-related IncF plasmids, pED208 and F. We identified several regions of intrinsic flexibility among the encoded T4SSs, as evidenced by partial or complete functionality of chimeric machines. Swapping of VirD4-like TraD type IV coupling proteins (T4CPs) yielded functional chimeras, indicative of relaxed specificity at the substrate-TraD and TraD-T4SS interfaces. Through mutational analyses, we further delineated domains of the TraD T4CPs contributing to recruitment of cognate vs heterologous DNA substrates. Remarkably, swaps of components comprising the outer membrane core complexes, a few F-specific subunits, or the TraA pilins supported DNA transfer in the absence of detectable pilus production. Among sequenced enterobacterial species in the NCBI database, we identified many strains that harbor two or more F-like plasmids and many F plasmids lacking one or more T4SS components required for self-transfer. We confirmed that host cells carrying co-resident, non-selftransmissible variants of pED208 and F elaborate chimeric T4SSs, as evidenced by transmission of both plasmids. We propose that T4SS plasticity enables the facile assembly of functional chimeras, and this intrinsic flexibility at the structural level can account for functional diversification of this superfamily over evolutionary time and, on a more immediate time-scale, to proliferation of transfer-defective MGEs in nature.

An archaeal nitrile hydratase from the halophilic archaeon A07HB70 exhibits high tolerance to 3-cyanopyridine and nicotinamide

Nitrile hydratase (NHase, EC 4.2.1.84) is widely used in the industrial production of biosynthetic amide compounds. NHases obtained from prokaryotic and eukaryotic sources have been widely studied, while the NHases derived from archaeal sources have not been reported. Here, we focused on a distinctive NHase derived from a halophilic archaeon (archaeon A07HB70, A.r NHase) that thrives in high-salt environments. A notable feature of this enzyme is the natural fusion of the α subunit with the activator. A.r NHase retained 89.14 % of its activity after exposure to 4.0 M substrate and 97.52 % of its activity after exposure to 4.0 M product. These findings indicate that A.r NHase exhibits significantly higher tolerance to both substrate and product compared to NHases derived from other sources, which may be due to its unique genetic structure. The investigation of such highly stable archaeal NHase can offer a theoretical foundation for modifying NHase derived from other sources. This, in turn, would enhance the potential industrial application of NHase.

Trade-offs constrain adaptive pathways to the type VI secretion system survival

The Type VI Secretion System (T6SS) is a nano-harpoon used by many bacteria to inject toxins into neighboring cells. While much is understood about mechanisms of T6SS-mediated toxicity, less is known about the ways that competitors can defend themselves against this attack, especially in the absence of their own T6SS. Here we subjected eight replicate populations of Escherichia coli to T6SS attack by Vibrio cholerae. Over ∼500 generations of competition, isolates of the E. coli populations evolved to survive T6SS attack an average of 27-fold better, through two convergently evolved pathways: apaH was mutated in six of the eight replicate populations, while the other two populations each had mutations in both yejM and yjeP. However, the mutations we identified are pleiotropic, reducing cellular growth rates, and increasing susceptibility to antibiotics and elevated pH. These trade-offs help us understand how the T6SS shapes the evolution of bacterial interactions.

Chimeric systems composed of swapped Tra subunits between distantly-related F plasmids reveal striking plasticity among type IV secretion machines

Bacterial type IV secretion systems (T4SSs) are a versatile family of macromolecular translocators, collectively able to recruit diverse DNA and protein substrates and deliver them to a wide range of cell types. Presently, there is little understanding of how T4SSs recognize substrate repertoires and form productive contacts with specific target cells. Although T4SSs are composed of a number of conserved subunits and adopt certain conserved structural features, they also display considerable compositional and structural diversity. Here, we explored the structural bases underlying the functional versatility of T4SSs through systematic deletion and subunit swapping between two conjugation systems encoded by the distantly-related IncF plasmids, pED208 and F. We identified several regions of intrinsic flexibility among the encoded T4SSs, as evidenced by partial or complete functionality of chimeric machines. Swapping of VirD4-like TraD type IV coupling proteins (T4CPs) yielded functional chimeras, indicative of relaxed specificity at the substrate - TraD and TraD - T4SS interfaces. Through mutational analyses, we further delineated domains of the TraD T4CPs contributing to recruitment of cognate vs heterologous DNA substrates. Remarkably, swaps of components comprising the outer membrane core complexes, a few F-specific subunits, or the TraA pilins supported DNA transfer in the absence of detectable pilus production. Among sequenced enterobacterial species in the NCBI database, we identified many strains that harbor two or more F-like plasmids and many F plasmids lacking one or more T4SS components required for self-transfer. We confirmed that host cells carrying co-resident, non-selftransmissible variants of pED208 and F elaborate chimeric T4SSs, as evidenced by transmission of both plasmids. We propose that T4SS plasticity enables the facile assembly of functional chimeras, and this intrinsic flexibility at the structural level can account for functional diversification of this superfamily over evolutionary time and, on a more immediate time-scale, to proliferation of transfer-defective MGEs in nature.

Conserved metabolic regulator ArcA responds to oxygen availability, iron limitation, and cell envelope perturbations during bacteremia

IMPORTANCE

Infections of the bloodstream are life-threatening and can result in sepsis. Gram-negative bacteria cause a significant portion of bloodstream infections, which is also referred to as bacteremia. The long-term goal of our work is to understand how such bacteria establish and maintain infection during bacteremia. We have previously identified the transcription factor ArcA, which promotes fermentation in bacteria, as a likely contributor to the growth and survival of bacteria in this environment. Here, we study ArcA in the Gram-negative species Citrobacter freundii, Klebsiella pneumoniae, and Serratia marcescens. Our findings aid in determining how these bacteria sense their environment, utilize nutrients, and generate energy while countering the host immune system. This information is critical for developing better models of infection to inform future therapeutic development.

Targeted mutagenesis and high-throughput screening of diversified gene and promoter libraries for isolating gain-of-function mutations

Targeted mutagenesis of a promoter or gene is essential for attaining new functions in microbial and protein engineering efforts. In the burgeoning field of synthetic biology, heterologous genes are expressed in new host organisms. Similarly, natural or designed proteins are mutagenized at targeted positions and screened for gain-of-function mutations. Here, we describe methods to attain complete randomization or controlled mutations in promoters or genes. Combinatorial libraries of one hundred thousands to tens of millions of variants can be created using commercially synthesized oligonucleotides, simply by performing two rounds of polymerase chain reactions. With a suitably engineered reporter in a whole cell, these libraries can be screened rapidly by performing fluorescence-activated cell sorting (FACS). Within a few rounds of positive and negative sorting based on the response from the reporter, the library can rapidly converge to a few optimal or extremely rare variants with desired phenotypes. Library construction, transformation and sequence verification takes 6-9 days and requires only basic molecular biology lab experience. Screening the library by FACS takes 3-5 days and requires training for the specific cytometer used. Further steps after sorting, including colony picking, sequencing, verification, and characterization of individual clones may take longer, depending on number of clones and required experiments.

Local Regulator AcrR Regulates Persister Formation by Repression of AcrAB Efflux Pump during Exponential Growth in Aeromonas veronii

Bacterial persisters refer to a small fraction of dormant variants that survive treatment with high concentrations of antibiotics. Increasing research indicates that multidrug efflux pumps play a major role in persister formation in many Gram-negative organisms. In the present study, the roles of the repressor of the AcrAB efflux pump, AcrR, in the regulation of the activity and function of the efflux, as well as in the production of persisters, were investigated in the pathogen Aeromonas veronii, which causes huge economic losses in the aquatic industry and threatens human health. We observed that exclusively in exponential-phase cells, not in stationary-phase cells, the deletion of the acrR gene significantly (P < 0.05) promoted the expression of the acrA and acrB genes and reduced the intracellular accumulation of the efflux substrate Hoechst 33342. Moreover, overexpression of acrR triggered decreased transcription of the promoter of the acrAB operon. The persister assay indicated that the loss of the AcrAB pump decreased the formation of persisters under challenge with all tested antibiotic types of chloramphenicol, fluoroquinolone, tetracycline, and β-lactam, while deletion of acrR caused an exponential-phase-specific increase in persister formation against chloramphenicol, tetracycline, and β-lactam. Our results provide molecular insights into the mechanism of bacterial persistence by demonstrating for the first time that the local regulator AcrR is involved in the modulation of persister formation in A. veronii through its repressive activity on the function of the AcrAB efflux pump during the exponential growth period.

D-Serine reduces the expression of the cytopathic genotoxin colibactin

Some Escherichia coli strains harbour the pks island, a 54 kb genomic island encoding the biosynthesis genes for a genotoxic compound named colibactin. In eukaryotic cells, colibactin can induce DNA damage, cell cycle arrest and chromosomal instability. Production of colibactin has been implicated in the development of colorectal cancer (CRC). In this study, we demonstrate the inhibitory effect of D-Serine on the expression of the pks island in both prototypic and clinically-associated colibactin-producing strains and determine the implications for cytopathic effects on host cells. We also tested a comprehensive panel of proteinogenic L-amino acids and corresponding D-enantiomers for their ability to modulate clbB transcription. Whilst several D-amino acids exhibited the ability to inhibit expression of clbB, D-Serine exerted the strongest repressing activity (>3.8-fold) and thus, we focussed additional experiments on D-Serine. To investigate the cellular effect, we investigated if repression of colibactin by D-Serine could reduce the cytopathic responses normally observed during infection of HeLa cells with pks + strains. Levels of γ-H2AX (a marker of DNA double strand breaks) were reduced 2.75-fold in cells infected with D-Serine treatment. Moreover, exposure of pks + E. coli to D-Serine during infection caused a reduction in cellular senescence that was observable at 72 h post infection. The recent finding of an association between pks-carrying commensal E. coli and CRC, highlights the necessity for the development of colibactin targeting therapeutics. Here we show that D-Serine can reduce expression of colibactin, and inhibit downstream cellular cytopathy, illuminating its potential to prevent colibactin-associated disease.

Critical role of the RpoE stress response pathway in polymyxin resistance of Escherichia coli

OBJECTIVES

Polymyxins, including colistin, are the drugs of last resort to treat MDR bacterial infections in humans. In-depth understanding of the molecular basis and regulation of polymyxin resistance would provide new therapeutic opportunities to combat increasing polymyxin resistance. Here we aimed to identify novel targets that are crucial for polymyxin resistance using Escherichia coli BL21(DE3), a unique colistin-resistant model strain.

METHODS

BL21(DE3) was subjected to random transposon mutagenesis for screening colistin-susceptible mutants. The insertion sites of desired mutants were mapped; the key genes of interest were also inactivated in different strains to examine functional conservation. Specific genes in the known PmrAB and PhoPQ regulatory network were inactivated to examine crosstalk among different pathways. Lipid A species and membrane phospholipids were analysed by normal phase LC/MS.

RESULTS

Among eight mutants with increased susceptibility to colistin, five mutants contained different mutations in three genes (rseP, degS and surA) that belong to the RpoE stress response pathway. Inactivation of rpoE, pmrB, eptA or pmrD led to significantly increased susceptibility to colistin; however, inactivation of phoQ or eptB did not change colistin MIC. RpoE mutation in different E. coli and Salmonella resistant strains all led to significant reduction in colistin MIC (16-32-fold). Inactivation of rpoE did not change the lipid A profile but significantly altered the phospholipid profile.

CONCLUSIONS

Inactivation of the important members of the RpoE regulon in polymyxin-resistant strains led to a drastic reduction in polymyxin MIC and an increase of lysophospholipids with no change in lipid A modifications.

Suppression of multiple phenotypic and RNA processing defects by over-expression of Oligoribonuclease in Escherichia coli

Oligoribonuclease (Orn) is an essential Ribonuclease (RNase) from Escherichia coli (E. coli), which plays a critical role in the conversion of short RNA molecules (NanoRNAs) to mononucleotides. Although no additional functions have been ascribed to Orn since its discovery nearly 50 years ago, it was observed in this study that the growth defects caused by a lack of two other RNases that do not digest NanoRNAs, PNPase and RNase PH, could be suppressed by Orn over-expression. Further analyses showed that Orn over-expression can alleviate the growth defects caused by an absence of other RNases even when its expression was increased by a small degree, and it can carry out molecular reactions that are normally performed by RNase T and RNase PH. In addition, biochemical assays revealed that Orn can fully digest single stranded RNAs within a variety of structural contexts. These studies provide new insights into Orn function and its ability to participate in multiple aspects of E. coli RNA metabolism.

Escherichia coli aceE variants coding pyruvate dehydrogenase improve the generation of pyruvate-derived acetoin

Several chromosomally expressed AceE variants were constructed in Escherichia coli ΔldhA ΔpoxB ΔppsA and compared using glucose as the sole carbon source. These variants were examined in shake flask cultures for growth rate, pyruvate accumulation, and acetoin production via heterologous expression of the budA and budB genes from Enterobacter cloacae ssp. dissolvens. The best acetoin-producing strains were subsequently studied in controlled batch culture at the one-liter scale. PDH variant strains attained up to four-fold greater acetoin than the strain expressing the wild-type PDH. In a repeated batch process, the H106V PDH variant strain attained over 43 g/L of pyruvate-derived products, acetoin (38.5 g/L) and 2R,3R-butanediol (5.0 g/L), corresponding to an effective concentration of 59 g/L considering the dilution. The acetoin yield from glucose was 0.29 g/g with a volumetric productivity of 0.9 g/L·h (0.34 g/g and 1.0 g/L·h total products). The results demonstrate a new tool in pathway engineering, the modification of a key metabolic enzyme to improve the formation of a product via a kinetically slow, introduced pathway. Direct modification of the pathway enzyme offers an alternative to promoter engineering in cases where the promoter is involved in a complex regulatory network.

Construction of a constitutively active type III secretion system for heterologous protein secretion

Proteins comprise a multibillion-dollar industry in enzymes and therapeutics, but bacterial protein production can be costly and inefficient. Proteins of interest (POIs) must be extracted from lysed cells and inclusion bodies, purified, and resolubilized, which adds significant time and cost to the protein-manufacturing process. The Salmonella pathogenicity island 1 (SPI-1) type III secretion system (T3SS) has been engineered to address these problems by secreting soluble, active proteins directly into the culture media, reducing the number of purification steps. However, the current best practices method of T3SS pathway activation is not ideal for industrial scaleup. Previously, the T3SS was activated by plasmid-based overexpression of the T3SS transcriptional regulator, hilA, which requires the addition of a small molecule inducer (IPTG) to the culture media. IPTG adds significant cost to production and plasmid-based expression is subject to instability in large-scale fermentation. Here, we modulate the upstream transcriptional regulator, hilD, to activate the T3SS via three distinct methods. In doing so, we develop a toolbox of T3SS activation methods and construct constitutively active T3SS strains capable of secreting a range of heterologous proteins at titers comparable to plasmid-based hilA overexpression. We also explore how each activation method in our toolbox impacts the SPI-1 regulatory cascade and discover an epistatic relationship between T3SS regulators, hilE and the hilD 3' untranslated region (hilD 3'UTR). Together, these findings further our goal of making an industrially competitive protein production strain that reduces the challenges associated with plasmid induction and maintenance. KEY POINTS: • Characterized 3 new type III secretion system (T3SS) activation methods for heterologous protein secretion, including 2 constitutive activation methods. • Eliminated the need for a second plasmid and a small molecule inducer to activate the system, making it more suitable for industrial production. • Discovered new regulatory insights into the SPI-1 T3SS, including an epistatic relationship between regulators hilE and the hilD 3' untranslated region.

Serial Recombineering Cloning to Build Selectable and Tagged Genomic P[acman] BAC Clones for Selection Transgenesis and Functional Gene Analysis using Drosophila melanogaster

Transgenes with genomic DNA fragments that encompass genes of interest are the gold standard for complementing null alleles in rescue experiments in the fruit fly Drosophila melanogaster. Of particular interest are genomic DNA clones available as bacterial artificial chromosomes (BACs) or fosmids from publicly available genomic DNA libraries. Genes contained within BAC and fosmid clones can be easily modified by recombineering cloning to insert peptide or protein tags to localize, visualize, or manipulate gene products, and to create point mutations or deletions for structure-function analysis of the inserted genes. However, since transgenesis efficiency is inversely correlated with transgene size, obtaining transgenic animals for increasingly larger BAC and fosmid clones requires increasingly laborious screening efforts using the transgenesis marker commonly used for these transgenes, the dominant eye color marker white+ . We recently described a drug-based selectable genetic platform for Drosophila melanogaster, which included four resistance markers that allow direct selection of transgenic animals, eliminating the need to identify transgenic progeny by laborious phenotypic screening. By integrating these resistance markers into BAC transgenes, we were able to isolate animals containing large transgenes by direct selection, avoiding laborious screening. Here we present procedures on how to upgrade BAC clones by serial recombineering cloning to build both selectable and tagged BAC transgenes, for selection transgenesis and functional gene analysis, respectively. We illustrate these procedures using a BAC clone encompassing the gene encoding the synaptic vesicle protein, cysteine string protein. We demonstrate that the modified BAC clone, serially recombineered with a selectable marker for selection transgenesis and an N-terminal green fluorescent protein tag for gene expression analysis, is functional by showing the expression pattern obtained after successful selection transgenesis. The protocols cover: (1) cloning and preparation of the recombineering templates needed for serial recombineering cloning to incorporate selectable markers and protein tags; (2) preparing electrocompetent cells needed to perform serial recombineering cloning; and (3) the serial recombineering workflow to generate both selectable and tagged genomic BAC reporter transgenes for selection transgenesis and functional gene analysis in Drosophila melanogaster. The protocols we describe can be easily adapted to incorporate any of four selectable markers, protein tags, or any other modification for structure-function analysis of the genes present within any of the BAC or fosmid clones. A protocol for generating transgenic animals using serially recombineered BAC clones is presented in an accompanying Current Protocols article (Venken, Matinyan, Gonzalez, & Dierick, 2023a). © 2023 Wiley Periodicals LLC. Basic Protocol 1: Cloning and preparation of recombineering templates used for serial recombineering cloning. Basic Protocol 2: Making electrocompetent cells of the bacterial strains used to perform serial recombineering cloning or induction of plasmid copy number. Basic Protocol 3: Serial recombineering cloning to generate both selectable and tagged genomic P[acman] BAC reporter transgenes for selection transgenesis and gene expression analysis in Drosophila melanogaster.

Recombineering in Non-Model Bacteria

The technology of recombineering, in vivo genetic engineering, was initially developed in Escherichia coli and uses bacteriophage-encoded homologous recombination proteins to efficiently recombine DNA at short homologies (35 to 50 nt). Because the technology is homology driven, genomic DNA can be modified precisely and independently of restriction site location. Recombineering uses linear DNA substrates that are introduced into the cell by electroporation; these can be PCR products, synthetic double-strand DNA (dsDNA), or single-strand DNA (ssDNA). Here we describe the applications, challenges, and factors affecting ssDNA and dsDNA recombineering in a variety of non-model bacteria, both Gram-negative and -positive, and recent breakthroughs in the field. We list different microbes in which the widely used phage λ Red and Rac RecET recombination systems have been used for in vivo genetic engineering. New homologous ssDNA and dsDNA recombineering systems isolated from non-model bacteria are also described. The Basic Protocol outlines a method for ssDNA recombineering in the non-model species of Shewanella. The Alternate Protocol describes the use of CRISPR/Cas as a counter-selection system in conjunction with recombineering to enhance recovery of recombinants. We provide additional background information, pertinent considerations for experimental design, and parameters critical for success. The design of ssDNA oligonucleotides (oligos) and various internet-based tools for oligo selection from genome sequences are also described, as is the use of oligo-mediated recombination. This simple form of genome editing uses only ssDNA oligo(s) and does not require an exogenous recombination system. The information presented here should help researchers identify a recombineering system suitable for their microbe(s) of interest. If no system has been characterized for a specific microbe, researchers can find guidance in developing a recombineering system from scratch. We provide a flowchart of decision-making paths for strategically applying annealase-dependent or oligo-mediated recombination in non-model and undomesticated bacteria. © 2022 Wiley Periodicals LLC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA. Basic Protocol: ssDNA recombineering in Shewanella species Alternate Protocol: ssDNA recombineering coupled to CRISPR/Cas9 in Shewanella species.

Profiling the intragenic toxicity determinants of toxin-antitoxin systems: revisiting hok/Sok regulation

Type I toxin-antitoxin systems (T1TAs) are extremely potent bacterial killing systems difficult to characterize using classical approaches. To assess the killing capability of type I toxins and to identify mutations suppressing the toxin expression or activity, we previously developed the FASTBAC-Seq (Functional AnalysiS of Toxin-Antitoxin Systems in BACteria by Deep Sequencing) method in Helicobacter pylori. This method combines a life and death selection with deep sequencing. Here, we adapted and improved our method to investigate T1TAs in the model organism Escherichia coli. As a proof of concept, we revisited the regulation of the plasmidic hok/Sok T1TA system. We revealed the death-inducing phenotype of the Hok toxin when it is expressed from the chromosome in the absence of the antitoxin and recovered previously described intragenic toxicity determinants of this system. We identified nucleotides that are essential for the transcription, translation or activity of Hok. We also discovered single-nucleotide substitutions leading to structural changes affecting either the translation or the stability of the hok mRNA. Overall, we provide the community with an easy-to-use approach to widely characterize TA systems from diverse types and bacteria.

Low levels of tetracyclines select for a mutation that prevents the evolution of high-level resistance to tigecycline

In a collection of Escherichia coli isolates, we discovered a new mechanism leading to frequent and high-level tigecycline resistance involving tandem gene amplifications of an efflux pump encoded by the tet(A) determinant. Some isolates, despite carrying a functional tet(A), could not evolve high-level tigecycline resistance by amplification due to the presence of a deletion in the TetR(A) repressor. This mutation impaired induction of tetA(A) (encoding the TetA(A) efflux pump) in presence of tetracyclines, with the strongest effect observed for tigecycline, subsequently preventing the development of tet(A) amplification-dependent high-level tigecycline resistance. We found that this mutated tet(A) determinant was common among tet(A)-carrying E. coli isolates and analysed possible explanations for this high frequency. First, while the mutated tet(A) was found in several ST-groups, we found evidence of clonal spread among ST131 isolates, which increases its frequency within E. coli databases. Second, evolution and competition experiments revealed that the mutation in tetR(A) could be positively selected over the wild-type allele at sub-inhibitory concentrations of tetracyclines. Our work demonstrates how low concentrations of tetracyclines, such as those found in contaminated environments, can enrich and select for a mutation that generates an evolutionary dead-end that precludes the evolution towards high-level, clinically relevant tigecycline resistance.

A study on evolution of antimicrobial resistance reveals how sub-inhibitory concentrations of antibiotics enrich and select for a mutated allele that prevents evolution towards clinically significant levels of antibiotic resistance.

A Novel Pathway of Chlorimuron-Ethyl Biodegradation by Chenggangzhangella methanolivorans Strain CHL1 and Its Molecular Mechanisms

Chlorimuron-ethyl is a widely used herbicide in agriculture. However, uncontrolled chlorimuron-ethyl application causes serious environmental problems. Chlorimuron-ethyl can be effectively degraded by microbes, but the underlying molecular mechanisms are not fully understood. In this study, we identified the possible pathways and key genes involved in chlorimuron-ethyl degradation by the Chenggangzhangella methanolivorans strain CHL1, a Methylocystaceae strain with the ability to degrade sulfonylurea herbicides. Using a metabolomics method, eight intermediate degradation products were identified, and three pathways, including a novel pyrimidine-ring-opening pathway, were found to be involved in chlorimuron-ethyl degradation by strain CHL1. Transcriptome sequencing indicated that three genes (atzF, atzD, and cysJ) are involved in chlorimuron-ethyl degradation by strain CHL1. The gene knock-out and complementation techniques allowed for the functions of the three genes to be identified, and the enzymes involved in the different steps of chlorimuron-ethyl degradation pathways were preliminary predicted. The results reveal a previously unreported pathway and the key genes of chlorimuron-ethyl degradation by strain CHL1, which have implications for attempts to enrich the biodegradation mechanism of sulfonylurea herbicides and to construct engineered bacteria in order to remove sulfonylurea herbicide residues from environmental media.

Editing of Phage Genomes—Recombineering-assisted SpCas9 Modification of Model Coliphages T7, T5, and T3

Abstract

Bacteriophages—viruses that infect bacterial cells—are the most abundant biological entities on Earth. The use of phages in fundamental research and industry requires tools for precise manipulation of their genomes. Yet, compared to bacterial genome engineering, modification of phage genomes is challenging because of the lack of selective markers and thus requires laborious screenings of recombinant/mutated phage variants. The development of the CRISPR-Cas technologies allowed to solve this issue by the implementation of negative selection that eliminates the parental phage genomes. In this manuscript, we summarize current methods of phage genome engineering and their coupling with CRISPR-Cas technologies. We also provide examples of our successful application of these methods for introduction of specific insertions, deletions, and point mutations in the genomes of model Escherichia coli lytic phages T7, T5, and T3.

High-Level Production of Catechol from Glucose by Engineered Escherichia coli

Catechol (CA) is an aromatic compound with important applications in the fine chemical and pharmaceutical fields. As an alternative strategy to petroleum-based chemical synthesis, the production of catechol by using microbial cell factories has attracted great interest. However, the toxicity of catechol to microbial cells significantly limits the efficient production of bio-based catechol via one-step fermentation. Therefore, in this study, a two-step strategy for the efficient synthesis of CA was designed. Protocatechuic acid (PCA) was first efficiently produced by the engineered Escherichia coli strain AAA01 via fermentation, and then PCA in the fermentative broth was converted into CA by the whole-cell biocatalyst AAA12 with PCA decarboxylase. By optimizing the expression of flavin isoprenyl transferases and protocatechuic acid decarboxylases, the titer of CA increased from 3.4 g/L to 15.8 g/L in 12 h through whole-cell biocatalysis, with a 365% improvement; after further optimizing the reaction conditions for whole-cell biocatalysis, the titer of CA achieved 17.7 g/L within 3 h, which is the highest titer reported so far. This work provides an effective strategy for the green biomanufacturing of toxic compounds by Escherichia coli cell factories.

The Strategy of Paratransgenesis for the Control of Malaria Transmission

Insect-borne diseases are responsible for important burdens on health worldwide particularly in Africa. Malaria alone causes close to half a million deaths every year, mostly in developing, tropical and subtropical countries, with 94% of the global deaths in 2019 occurring in the WHO African region. With several decades, vector control measures have been fundamental to fight against malaria. Considering the spread of resistance to insecticides in mosquitoes and to drugs in parasites, the need for novel strategies to inhibit the transmission of the disease is pressing. In recent years, several studies have focused on the interaction of malaria parasites, bacteria and their insect vectors. Their findings suggested that the microbiota of mosquitoes could be used to block Plasmodium transmission. A strategy, termed paratransgenesis, aims to interfere with the development of malaria parasites within their vectors through genetically-modified microbes, which produce antimalarial effectors inside the insect host. Here we review the progress of the paratransgenesis approach. We provide a historical perspective and then focus on the choice of microbial strains and on genetic engineering strategies. We finally describe the different steps from laboratory design to field implementation to fight against malaria.

Vertex protein PduN tunes encapsulated pathway performance by dictating bacterial metabolosome morphology

Engineering subcellular organization in microbes shows great promise in addressing bottlenecks in metabolic engineering efforts; however, rules guiding selection of an organization strategy or platform are lacking. Here, we study compartment morphology as a factor in mediating encapsulated pathway performance. Using the 1,2-propanediol utilization microcompartment (Pdu MCP) system from Salmonella enterica serovar Typhimurium LT2, we find that we can shift the morphology of this protein nanoreactor from polyhedral to tubular by removing vertex protein PduN. Analysis of the metabolic function between these Pdu microtubes (MTs) shows that they provide a diffusional barrier capable of shielding the cytosol from a toxic pathway intermediate, similar to native MCPs. However, kinetic modeling suggests that the different surface area to volume ratios of MCP and MT structures alters encapsulated pathway performance. Finally, we report a microscopy-based assay that permits rapid assessment of Pdu MT formation to enable future engineering efforts on these structures.

Morphology of metabolosomes affects the encapsulated pathway performance. Here, the authors combine experimental characterizations with structural and kinetic modeling to reveal how the shell protein PduN changes the morphology of 1,2-propanediol utilization (Pdu) metabolosome and how this morphology shift impacts Pdu function.

Bioproduction of Linalool From Paper Mill Waste

A key challenge in chemicals biomanufacturing is the maintenance of stable, highly productive microbial strains to enable cost-effective fermentation at scale. A “cookie-cutter” approach to microbial engineering is often used to optimize host stability and productivity. This can involve identifying potential limitations in strain characteristics followed by attempts to systematically optimize production strains by targeted engineering. Such targeted approaches however do not always lead to the desired traits. Here, we demonstrate both ‘hit and miss’ outcomes of targeted approaches in attempts to generate a stable Escherichia coli strain for the bioproduction of the monoterpenoid linalool, a fragrance molecule of industrial interest. First, we stabilized linalool production strains by eliminating repetitive sequences responsible for excision of pathway components in plasmid constructs that encode the pathway for linalool production. These optimized pathway constructs were then integrated within the genome of E. coli in three parts to eliminate a need for antibiotics to maintain linalool production. Additional strategies were also employed including: reduction in cytotoxicity of linalool by adaptive laboratory evolution and modification or homologous gene replacement of key bottleneck enzymes GPPS/LinS. Our study highlights that a major factor influencing linalool titres in E. coli is the stability of the genetic construct against excision or similar recombination events. Other factors, such as decreasing linalool cytotoxicity and changing pathway genes, did not lead to improvements in the stability or titres obtained. With the objective of reducing fermentation costs at scale, the use of minimal base medium containing paper mill wastewater secondary paper fiber as sole carbon source was also investigated. This involved simultaneous saccharification and fermentation using either supplemental cellulase blends or by co-expressing secretable cellulases in E. coli containing the stabilized linalool production pathway. Combined, this study has demonstrated a stable method for linalool production using an abundant and low-cost feedstock and improved production strains, providing an important proof-of-concept for chemicals production from paper mill waste streams. For scaled production, optimization will be required, using more holistic approaches that involve further rounds of microbial engineering and fermentation process development.

Homologs of the Escherichia coli F Element Protein TraR, Including Phage Lambda Orf73, Directly Reprogram Host Transcription

Bacterial cells and their associated plasmids and bacteriophages encode numerous small proteins of unknown function. One example, the 73-amino-acid protein TraR, is encoded by the transfer operon of the conjugative F plasmid of Escherichia coli. TraR is a distant homolog of DksA, a protein found in almost all proteobacterial species that is required for ppGpp to regulate transcription during the stringent response. TraR and DksA increase or decrease transcription initiation depending on the kinetic features of the promoter by binding directly to RNA polymerase without binding to DNA. Unlike DksA, whose full activity requires ppGpp as a cofactor, TraR is fully active by itself and unaffected by ppGpp. TraR belongs to a family of divergent proteins encoded by proteobacterial bacteriophages and other mobile elements. Here, we experimentally addressed whether other members of the TraR family function like the F element-encoded TraR. Purified TraR and all 5 homologs that were examined bound to RNA polymerase, functioned at lower concentrations than DksA, and complemented a dksA-null strain for growth on minimal medium. One of the homologs, λ Orf73, encoded by bacteriophage lambda, was examined in greater detail. λ Orf73 slowed host growth and increased phage burst size. Mutational analysis suggested that λ Orf73 and TraR have a similar mechanism for inhibiting rRNA and r-protein promoters. We suggest that TraR and its homologs regulate host transcription to divert cellular resources to phage propagation or conjugation without induction of ppGpp and a stringent response.

Recent advances in high-throughput metabolic engineering: Generation of oligonucleotide-mediated genetic libraries

The preparation of genetic libraries is an essential step to evolve microorganisms and study genotype-phenotype relationships by high-throughput screening/selection. As the large-scale synthesis of oligonucleotides becomes easy, cheap, and high-throughput, numerous novel strategies have been developed in recent years to construct high-quality oligo-mediated libraries, leveraging state-of-art molecular biology tools for genome editing and gene regulation. This review presents an overview of recent advances in creating and characterizing in vitro and in vivo genetic libraries, based on CRISPR/Cas, regulatory RNAs, and recombineering, primarily for Escherichia coli and Saccharomyces cerevisiae. These libraries' applications in high-throughput metabolic engineering, strain evolution and protein engineering are also discussed.

The Atypical Antipsychotic Quetiapine Promotes Multiple Antibiotic Resistance in Escherichia coli

Atypical antipsychotic (AAP) medication is a critical tool for treating symptoms of psychiatric disorders. While AAPs primarily target dopamine (D2) and serotonin (5HT2A and 5HT1A) receptors, they also exhibit intrinsic antimicrobial activity as an off-target effect. Because AAPs are often prescribed to patients for many years, a potential risk associated with long-term AAP use is the unintended emergence of bacteria with antimicrobial resistance (AMR). Here, we show that exposure to the AAP quetiapine at estimated gut concentrations promotes AMR in Escherichia coli after 6 weeks. Quetiapine-exposed isolates exhibited an increase in MICs for ampicillin, tetracycline, ceftriaxone, and levofloxacin. By whole-genome sequencing analysis, we identified mutations in genes that confer AMR, including the repressor for the multiple antibiotic resistance mar operon (marR), and real-time reverse transcription-quantitative PCR (RT-qPCR) analysis showed increased levels of marA, acrA, and tolC mRNAs and reduced levels of ompF mRNA in the isolates carrying marR mutations. To determine the contribution of each marR mutation to AMR, we constructed isogenic strains carrying individual mutant marR alleles in the parent background and reevaluated their resistance phenotypes using MIC and RT-qPCR assays. While marR mutations induced robust activity of the mar operon, they resulted in only modest increases in MICs. Interestingly, although these marR mutations did not fully recapitulate the AMR phenotype of the quetiapine-exposed isolates, we show that marR mutations promote growth fitness in the presence of quetiapine. Our findings revealed an important link between the use of AAPs and AMR development in E. coli. IMPORTANCE AAP medication is a cornerstone in the treatment of serious psychiatric disease. The AAPs are known to exhibit antimicrobial activity; therefore, a potential unintended risk of long-term AAP use may be the emergence of AMR, although such risk has received little attention. In this study, we describe the development of multidrug antibiotic resistance in Escherichia coli after 6 weeks of exposure to the AAP quetiapine. Investigation of mutations in the marR gene, which encodes a repressor for the multiple antibiotic resistance (mar) operon, reveals a potential mechanism that increases the fitness of E. coli in the presence of quetiapine. Our findings establish a link between the use of AAPs and AMR development in bacteria.

Identification of distinct capsule types associated with Serratia marcescens infection isolates

Serratia marcescens is a versatile opportunistic pathogen that can cause a variety of infections, including bacteremia. Our previous work established that the capsule polysaccharide (CPS) biosynthesis and translocation locus contributes to the survival of S. marcescens in a murine model of bacteremia and in human serum. In this study, we determined the degree of capsule genetic diversity among S. marcescens isolates. Capsule loci (KL) were extracted from >300 S. marcescens genome sequences and compared. A phylogenetic comparison of KL sequences demonstrated a substantial level of KL diversity within S. marcescens as a species and a strong delineation between KL sequences originating from infection isolates versus environmental isolates. Strains from five of the identified KL types were selected for further study and electrophoretic analysis of purified CPS indicated the production of distinct glycans. Polysaccharide composition analysis confirmed this observation and identified the constituent monosaccharides for each strain. Two predominant infection-associated clades, designated KL1 and KL2, emerged from the capsule phylogeny. Bacteremia strains from KL1 and KL2 were determined to produce ketodeoxynonulonic acid and N-acetylneuraminic acid, two sialic acids that were not found in strains from other clades. Further investigation of KL1 and KL2 sequences identified two genes, designated neuA and neuB, that were hypothesized to encode sialic acid biosynthesis functions. Disruption of neuB in a KL1 isolate resulted in the loss of sialic acid and CPS production. The absence of sialic acid and CPS production also led to increased susceptibility to internalization by a human monocytic cell line, demonstrating that S. marcescens phagocytosis resistance requires CPS. Together, these results establish the capsule genetic repertoire of S. marcescens and identify infection-associated clades with sialic acid CPS components.

An attenuated multiple genetic mutant of Mycoplasma pneumoniae imparts good immuno-protection against M. pneumoniae pneumonia in BALB/c mice

Mycoplasma pneumoniae (M. pneumoniae) is the causative agent of both upper and lower respiratory infections that can lead to pneumonia, extrapulmonary complications and devastating sequela. With the increasing rate of macrolide-resistant strains, the severe clinical consequence of refractory mycoplasma pneumonia in children health calls for the need of vaccine research for this pathogen. In this report, the immunomodulatory effectiveness of a live attenuated M. pneumoniae vaccine was evaluated. The vaccine strain was a mutant strain of M. pneumoniae, MUT129, obtained after multiple passages of M129 strain in PPLO broth. The SNP/InDel detection results showed that mutations were present in genes encoding the adhesion organelle-associated proteins and lipoproteins of M. pneumoniae MUT129. Upon intranasal challenge of BALB/c mice with 1 × 10⁷ CFU of MUT129, there were very small amount of Mycoplasma antigens and almost no M. pneumoniae present in the lung tissues of BALB/c mice. Besides, there was almost no inflammatory cell infiltration in the lung tissue. Results of the M. pneumoniae challenge study showed that mice immunized with MUT129 presented with less inflammation, lower detectable number of M. pneumoniae in the lungs when compared with the unimmunized mice. These results indicated that the live attenuated vaccine can efficiently prevent the proliferation of M. pneumonia in the lungs, reduce but not completely prevent the pulmonary inflammatory response.

The TelN/tos-assisted Precise Targeting of Chromosome Segments (TAPE)

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We first apply the TelN/tos system to target genomic segments in E. coli host.

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We successfully cloned the targeted bacterial DNA fragment up to 156 kb.

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The TAPE method takes no more than five days to directly obtain the large DNA sequence.

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The TAPE method has no preferences on genome sequence.

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The results showed a considerable improvement of cloning efficiency.

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The TAPE method provides a powerful tool to support the study on synthetic biology.

Introduction

Performing genomic large segmentation experiments will promote the annotation of complex genomic functions and contribute to the synthesis of designed genomes. It is challenging to obtain and manipulate large or complex DNA sequences with high efficiency.

Objectives

This study aims to develop an effective method for direct cloning of target genome sequences from different species.

Methods

The TelN/tos system and a linear plasmid vector were first used to directly clone the large genomic segments in E. coli. For the in vitro cloning reaction, two telomeric sites were developed using TelN protelomerase at the end of the linear plasmid vector. The target DNA sequence can be easily hooked with the homology arms and maintained as a linear artificial chromosome with arbitrary restriction sites in a specific E. coli strain.

Results

Using the linear cloning strategy, we successfully cloned the bacterial DNA fragment of 156 kb, a yeast genomic fragment of 124 kb and mammalian mitochondrial fragment of 16 kb. The results showed a considerable improvement in cloning efficiency and demonstrated the important role of vector ratio in the cloning process.

Conclusion

Due to the high efficiency and stability, TAPE is an effective technique for DNA cloning and fundamental molecular biotechnology method in synthetic biology.

Structure of a type IV secretion system core complex encoded by multi-drug resistance F plasmids

Bacterial type IV secretion systems (T4SSs) are largely responsible for the proliferation of multi-drug resistance. We solved the structure of the outer-membrane core complex (OMCC_F) of a T4SS encoded by a conjugative F plasmid at <3.0 Å resolution by cryoelectron microscopy. The OMCC_F consists of a 13-fold symmetrical outer ring complex (ORC) built from 26 copies of TraK and TraV C-terminal domains, and a 17-fold symmetrical central cone (CC) composed of 17 copies of TraB β-barrels. Domains of TraV and TraB also bind the CC and ORC substructures, establishing that these proteins undergo an intraprotein symmetry alteration to accommodate the C13:C17 symmetry mismatch. We present evidence that other pED208-encoded factors stabilize the C13:C17 architecture and define the importance of TraK, TraV and TraB domains to T4SS_F function. This work identifies OMCC_F structural motifs of proposed importance for structural transitions associated with F plasmid dissemination and F pilus biogenesis.

Expression of the DeaD RNA helicase is regulated at multiple levels through its long mRNA 5' untranslated region

DEAD-box proteins (DBPs) are a prominent class of RNA remodeling proteins that alter RNA structure, a process they typically perform through an ATP-dependent RNA helicase activity. Although many DBPs have been characterized at the structural and functional level in detail, much less is known about how they are regulated. We previously showed that the messenger RNA (mRNA) for the Escherichia coli (E. coli) DeaD DBP contains an unusually long 5' untranslated region (5' UTR) of 838 nucleotides (nts) and that it is the primary RNA determinant of DeaD autoregulation. We speculated that such a long and complex 5' UTR might regulate deaD expression in additional ways. Here we show that the deaD mRNA 5' UTR regulates deaD expression at two additional levels: temperature dependent expression and through a stem-loop structure overlapping the start codon. These results support the hypothesis that a long 5' UTR can regulate gene expression through multiple mechanisms. Importance The expression of genes is frequently regulated by determinants with the 5' UTR. Although many different regulatory mechanisms that operate via the 5' UTR have been described, the functional relevance of genes with long UTRs is less clear. Here, we show that the 838 nt long 5' UTR in the deaD mRNA regulates the expression of DeaD at multiple levels. We propose that long UTRs originate to provide precise control of gene expression through multiple regulatory mechanisms, and they are indicators of the importance of their associated gene products for cellular adaptation to different environments.

Density-based binning of gene clusters to infer function or evolutionary history using GeneGrouper

MOTIVATION

Identifying variant forms of gene clusters of interest in phylogenetically proximate and distant taxa can help to infer their evolutionary histories and functions. Conserved gene clusters may differ by only a few genes, but these small differences can in turn induce substantial phenotypes, such as by the formation of pseudogenes or insertions interrupting regulation. Particularly as microbial genomes and metagenomic assemblies become increasingly abundant, unsupervised grouping of similar, but not necessarily identical, gene clusters into consistent bins can provide a population-level understanding of their gene content variation and functional homology.

RESULTS

We developed GeneGrouper, a command-line tool that uses a density-based clustering method to group gene clusters into bins. GeneGrouper demonstrated high recall and precision in benchmarks for the detection of the 23-gene Salmonella enterica LT2 Pdu gene cluster and four-gene Pseudomonas aeruginosa PAO1 Mex gene cluster among 435 genomes spanning mixed taxa. In a subsequent application investigating the diversity and impact of gene-complete and -incomplete LT2 Pdu gene clusters in 1130 S.enterica genomes, GeneGrouper identified a novel, frequently occurring pduN pseudogene. When investigated in vivo, introduction of the pduN pseudogene negatively impacted microcompartment formation. We next demonstrated the versatility of GeneGrouper by clustering distant homologous gene clusters and variable gene clusters found in integrative and conjugative elements.

AVAILABILITY AND IMPLEMENTATION

GeneGrouper software and code are publicly available at https://pypi.org/project/GeneGrouper/.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Plant Gene Modification by BAC Recombineering

Recombineering approaches exploiting the bacteriophage λ Red recombination functions are widely used for versatile modification of eukaryotic genes carried by bacterial artificial chromosomes (BACs) in E. coli. Whereas BAC transformation provides a simple means for integration of modified genes into the genomes of animal cells to generate knock-in and knockout lines, successful application of this strategy is hampered by low frequency of homologous recombination in higher plants. However, plant cells can be transformed at a high frequency using the transferred DNA (T-DNA) of Agrobacterium, which is stably and randomly integrated into the plant genome. The function of plant genes that are modified by recombineering and transferred by Agrobacterium T-DNA vectors into plant cells can thus be suitably studied using genetic complementation of knockout mutations induced by either T-DNA insertions or genome editing with T-DNA-based Crisp/Cas9 constructs. Here we describe two recombineering protocols for modification and transfer of plant genes from BACs into Agrobacterium T-DNA plant transformation vectors. The first protocol uses a conditional suicide ccdB gene cassette to assist the genetic complementation assays by generation of point mutations, deletions, and insertions at any gene position. The second "turbo"-recombineering protocol exploits various I-SceI insertion cassettes for fusing of fluorescent protein tags to the plant gene products to facilitate the characterization of their in vivo interacting partners by affinity purification, mass spectrometry, and cellular localization studies.

Targeted chromosomal Escherichia coli:dnaB exterior surface residues regulate DNA helicase behavior to maintain genomic stability and organismal fitness

Helicase regulation involves modulation of unwinding speed to maintain coordination of DNA replication fork activities and is vital for replisome progression. Currently, mechanisms for helicase regulation that involve interactions with both DNA strands through a steric exclusion and wrapping (SEW) model and conformational shifts between dilated and constricted states have been examined in vitro. To better understand the mechanism and cellular impact of helicase regulation, we used CRISPR-Cas9 genome editing to study four previously identified SEW-deficient mutants of the bacterial replicative helicase DnaB. We discovered that these four SEW mutations stabilize constricted states, with more fully constricted mutants having a generally greater impact on genomic stress, suggesting a dynamic model for helicase regulation that involves both excluded strand interactions and conformational states. These dnaB mutations result in increased chromosome complexities, less stable genomes, and ultimately less viable and fit strains. Specifically, dnaB:mut strains present with increased mutational frequencies without significantly inducing SOS, consistent with leaving single-strand gaps in the genome during replication that are subsequently filled with lower fidelity. This work explores the genomic impacts of helicase dysregulation in vivo, supporting a combined dynamic regulatory mechanism involving a spectrum of DnaB conformational changes and relates current mechanistic understanding to functional helicase behavior at the replication fork.

DNA replication is a vital biological process, and the proteins involved are structurally and functionally conserved across all domains of life. As our fundamental knowledge of genes and genetics grows, so does our awareness of links between acquired genetic mutations and disease. Understanding how genetic material is replicated accurately and efficiently and with high fidelity is the foundation to identifying and solving genome-based diseases. E. coli are model organisms, containing core replisome proteins, but lack the complexity of the human replication system, making them ideal for investigating conserved replisome behaviors. The helicase enzyme acts at the forefront of the replication fork to unwind the DNA helix and has also been shown to help coordinate other replisome functions. In this study, we examined specific mutations in the helicase that have been shown to regulate its conformation and speed of unwinding. We investigate how these mutations impact the growth, fitness, and cellular morphology of bacteria with the goal of understanding how helicase regulation mechanisms affect an organism’s ability to survive and maintain a stable genome.

Morphological and Transcriptional Responses to CRISPRi Knockdown of Essential Genes in Escherichia coli

CRISPR interference (CRISPRi) has facilitated the study of essential genes in diverse organisms using both high-throughput and targeted approaches. Despite the promise of this technique, no comprehensive arrayed CRISPRi library targeting essential genes exists for the model bacterium Escherichia coli, or for any Gram-negative species. Here, we built and characterized such a library. Each of the ∼500 strains in our E. coli library contains an inducible, chromosomally integrated single guide RNA (sgRNA) targeting an essential (or selected nonessential) gene and can be mated with a pseudo-Hfr donor strain carrying a dcas9 cassette to create a CRISPRi knockdown strain. Using this system, we built an arrayed library of CRISPRi strains and performed population and single-cell growth and morphology measurements as well as targeted follow-up experiments. These studies found that inhibiting translation causes an extended lag phase, identified new modulators of cell morphology, and revealed that the morphogene mreB is subject to transcriptional feedback regulation, which is critical for the maintenance of morphology. Our findings highlight canonical and noncanonical roles for essential genes in numerous aspects of cellular homeostasis. IMPORTANCE Essential genes make up only ∼5 to 10% of the genetic complement in most organisms but occupy much of their protein synthesis and account for almost all antibiotic targets. Despite the importance of essential genes, their intractability has, until recently, hampered efforts to study them. CRISPRi has facilitated the study of essential genes by allowing inducible and titratable depletion. However, all large-scale CRISPRi studies in Gram-negative bacteria thus far have used plasmids to express CRISPRi components and have been constructed in pools, limiting their utility for targeted assays and complicating the determination of antibiotic effects. Here, we use a modular method to construct an arrayed library of chromosomally integrated CRISPRi strains targeting the essential genes of the model bacterium Escherichia coli. This library enables targeted studies of essential gene depletions and high-throughput determination of antibiotic targets and facilitates studies targeting the outer membrane, an essential component that serves as the major barrier to antibiotics.

Goodbye PAM: Phage λ's Red recombination system cripples PAMs and helps dodge CRISPR attacks

Ever wondered how the phage λ Red recombination system resembles the Red Queen? Hossain et al. (2021) provide an answer in this issue of Cell Host & Microbe. They show that Red debilitates PAM sequences by mutagenic repair of CRISPR-targeted DNA breaks in infecting λ, thus shaping the phage-CRISPR arms race.

Adaptive Optimization Boosted the Production of Moenomycin A in the Microbial Chassis Streptomyces albus J1074

Great efforts have been made to improve Streptomyces chassis for efficient production of targeted natural products. Moenomycin family antibiotics, represented by moenomycin (Moe) and nosokomycin, are phosphoglycolipid antibiotics that display extraordinary inhibition against Gram-positive bacteria. Herein, we assembled a completed 34 kb hybrid biosynthetic gene cluster (BGC) of moenomycin A (moe-BGC) based on a 24 kb nosokomycin analogue biosynthetic gene cluster (noso-BGC). The heterologous expression of the hybrid moe-BGC in Streptomyces albus J1074 achieved the production of moenomycin A in the recombinant strain LX01 with a yield of 12.1 ± 2 mg/L. Further strong promoter refactoring to improve the transcriptional levels of all of the functional genes in strain LX02 enhanced the production of moenomycin A by 58%. However, the yield improvement of moenomycin A resulted in a dramatic 38% decrease in the chassis biomass compared with the control strain. To improve the weak physiological tolerance to moenomycin A of the chassis, another copy of the gene salb-PBP2 (P238N&F200D), encoding peptidoglycan biosynthetic protein PBP2, was introduced into the chassis strain, producing strain LX03. Cell growth was restored, and the fermentation titer of moenomycin A was 130% higher than that of LX01. Additionally, the production of moenomycin A in strain LX03 was further elevated by 45% to 40.0 ± 3 mg/L after media optimization. These results suggested that the adaptive optimization strategy of strong promoter refactoring in the BGC plus physiological tolerance in the chassis was an efficient approach for obtaining the desired natural products with high titers.

Bacteriophage λ RexA and RexB Functions Assist the Transition from Lysogeny to Lytic Growth

The CI and Cro repressors of bacteriophage λ create a bistable switch between lysogenic and lytic growth. In λ lysogens, CI repressor expressed from the P_RM promoter blocks expression of the lytic promoters P_L and P_R to allow stable maintenance of the lysogenic state. When lysogens are induced, CI repressor is inactivated and Cro repressor is expressed from the lytic P_R promoter. Cro repressor blocks P_RM transcription and CI repressor synthesis to ensure that the lytic state proceeds. RexA and RexB proteins, like CI, are expressed from the P_RM promoter in λ lysogens; RexB is also expressed from a second promoter, P_LIT , embedded in rexA. Here we show that RexA binds CI repressor and assists the transition from lysogenic to lytic growth, using both intact lysogens and defective prophages with reporter genes under control of the lytic P_L and P_R promoters. Once lytic growth begins, if the bistable switch does return to the immune state, RexA expression lessens the probability that it will remain there, thus stabilizing the lytic state and activation of the lytic P_L  and P_R  promoters. RexB modulates the effect of RexA and may also help establish phage DNA replication as lytic growth ensues.

Development of a BoHV-4 viral vector expressing tgD of BoHV-1 and evaluation of its immunogenicity in mouse model

In recent years, Bovine herpesvirus 4 (BoHV-4) has emerged as an attractive gene delivery viral vector, mainly for vaccination purposes in the veterinary field. In the present study, a new infectious clone of the BoHV-4 genome carrying a bacterial artificial chromosome vector (BoHV-4-BAC) was developed by homologous recombination in mammalian cell culture and bacterial systems, and exploited to express a truncated form of glycoprotein D (tgD) of Bovine herpesvirus 1 (BoHV-1) (BoHV-4-tgD∆TK) as a vaccine candidate. This construct's immunogenicity was compared to a DNA vector expressing the same antigen (pC-tgD) in a BALB/c mouse model. After the mice were immunized, total and specific antibody responses, cytokine responses, total splenocyte cells proliferation/cytotoxicity, and virus neutralization assays were conducted to analyze the immune response elicited by both constructs. Mice from both vaccine groups developed significant humoral and cellular immune responses after a booster dose regime was conducted on day 28 post-injection. In almost all immunological assays, BoHV-4-tgDΔTK induced as high an immune response as pC-tgD. In both vaccine constructs, neutralizing antibodies were a significant determining factor in protection against BoHV-1, even after the first injection. We conclude that a BoHV-4-based viral vector offers an effective immunization strategy as an alternative to DNA-based immunization platforms, at least to combat BoHV-1.

Loss of histone methyltransferase ASH1L in the developing mouse brain causes autistic-like behaviors

Autism spectrum disorder (ASD) is a neurodevelopmental disease associated with various gene mutations. Recent genetic and clinical studies report that mutations of the epigenetic gene ASH1L are highly associated with human ASD and intellectual disability (ID). However, the causality and underlying molecular mechanisms linking ASH1L mutations to genesis of ASD/ID remain undetermined. Here we show loss of ASH1L in the developing mouse brain is sufficient to cause multiple developmental defects, core autistic-like behaviors, and impaired cognitive memory. Gene expression analyses uncover critical roles of ASH1L in regulating gene expression during neural cell development. Thus, our study establishes an ASD/ID mouse model revealing the critical function of an epigenetic factor ASH1L in normal brain development, a causality between Ash1L mutations and ASD/ID-like behaviors in mice, and potential molecular mechanisms linking Ash1L mutations to brain functional abnormalities.

Pyruvate Production by Escherichia coli by Use of Pyruvate Dehydrogenase Variants

Altering metabolic flux at a key branch point in metabolism has commonly been accomplished through gene knockouts or by modulating gene expression. An alternative approach to direct metabolic flux preferentially toward a product is decreasing the activity of a key enzyme through protein engineering. In Escherichia coli, pyruvate can accumulate from glucose when carbon flux through the pyruvate dehydrogenase complex is suppressed. Based on this principle, 16 chromosomally expressed AceE variants were constructed in E. coli C and compared for growth rate and pyruvate accumulation using glucose as the sole carbon source. To prevent conversion of pyruvate to other products, the strains also contained deletions in two nonessential pathways: lactate dehydrogenase (ldhA) and pyruvate oxidase (poxB). The effect of deleting phosphoenolpyruvate synthase (ppsA) on pyruvate assimilation was also examined. The best pyruvate-accumulating strains were examined in controlled batch and continuous processes. In a nitrogen-limited chemostat process at steady-state growth rates of 0.15 to 0.28 h-1, an engineered strain expressing the AceE[H106V] variant accumulated pyruvate at a yield of 0.59 to 0.66 g pyruvate/g glucose with a specific productivity of 0.78 to 0.92 g pyruvate/g cells·h. These results provide proof of concept that pyruvate dehydrogenase complex variants can effectively shift carbon flux away from central carbon metabolism to allow pyruvate accumulation. This approach can potentially be applied to other key enzymes in metabolism to direct carbon toward a biochemical product. IMPORTANCE Microbial production of biochemicals from renewable resources has become an efficient and cost-effective alternative to traditional chemical synthesis methods. Metabolic engineering tools are important for optimizing a process to perform at an economically feasible level. This study describes an additional tool to modify central metabolism and direct metabolic flux to a product. We have shown that variants of the pyruvate dehydrogenase complex can direct metabolic flux away from cell growth to increase pyruvate production in Escherichia coli. This approach could be paired with existing strategies to optimize metabolism and create industrially relevant and economically feasible processes.

Dynamic Control of Gene Expression with Riboregulated Switchable Feedback Promoters

One major challenge in synthetic biology is the deleterious impacts of cellular stress caused by expression of heterologous pathways, sensors, and circuits. Feedback control and dynamic regulation are broadly proposed strategies to mitigate this cellular stress by optimizing gene expression levels temporally and in response to biological cues. While a variety of approaches for feedback implementation exist, they are often complex and cannot be easily manipulated. Here, we report a strategy that uses RNA transcriptional regulators to integrate additional layers of control over the output of natural and engineered feedback responsive circuits. Called riboregulated switchable feedback promoters (rSFPs), these gene expression cassettes can be modularly activated using multiple mechanisms, from manual induction to autonomous quorum sensing, allowing control over the timing, magnitude, and autonomy of expression. We develop rSFPs in Escherichia coli to regulate multiple feedback networks and apply them to control the output of two metabolic pathways. We envision that rSFPs will become a valuable tool for flexible and dynamic control of gene expression in metabolic engineering, biological therapeutic production, and many other applications.