A new logic for DNA engineering using recombination in Escherichia coli

Proof of ssDNA degraded from dsDNA for ET recombination

The widely used ET recombination requires an ssDNA product degraded by Rac phage protein E588 from dsDNA for strand invasion. However, proof of the ssDNA product is still elusive. The study provided three levels of proof sequentially. The probable ssDNAs degraded by E588 from the fluorescent plus-, minus-, or double-stranded dsDNA pET28a-xylanase exhibited a half fluorescence intensity of the corresponding dsDNAs, equivalent to the E588 degradation nucleotides half that of the total nucleotides degraded from the corresponding dsDNA. The ssDNA product degraded by E588 from the fluorescent minus-stranded dsDNA was confirmed by gradient gel-electrophoresis and two nuclease degradation reactions. Degraded by E588 from the dsDNA pET28a-xylanase that had a phosphorothioated plus-stranded 5'-terminus, the plus-stranded ssDNA product was separated via gel electrophoresis and recovered via a DNAclean kit. The recovered ssDNA product was proven to have intact 5'- and 3'-ends by DNA sequencing analysis. This study provides a solid foundation for the mechanism of ssDNA invasion.

Engineered bacteriophages: A panacea against pathogenic and drug resistant bacteria

Antimicrobial resistance (AMR) is a major global concern; antibiotics and other regular treatment methods have failed to overcome the increasing number of infectious diseases. Bacteriophages (phages) are viruses that specifically target/kill bacterial hosts without affecting other human microbiome. Phage therapy provides optimism in the current global healthcare scenario with a long history of its applications in humans that has now reached various clinical trials. Phages in clinical trials have specific requirements of being exclusively lytic, free from toxic genes with an enhanced host range that adds an advantage to this requisite. This review explains in detail the various phage engineering methods and their potential applications in therapy. To make phages more efficient, engineering has been attempted using techniques like conventional homologous recombination, Bacteriophage Recombineering of Electroporated DNA (BRED), clustered regularly interspaced short palindromic repeats (CRISPR)-Cas, CRISPY-BRED/Bacteriophage Recombineering with Infectious Particles (BRIP), chemically accelerated viral evolution (CAVE), and phage genome rebooting. Phages are administered in cocktail form in combination with antibiotics, vaccines, and purified proteins, such as endolysins. Thus, phage therapy is proving to be a better alternative for treating life-threatening infections, with more specificity and fewer detrimental consequences.

Construction of Anthocyanin Biosynthesis System Using Chalcone as a Substrate in Lactococcus lactis NZ9000

Anthocyanins are high-value natural compounds, but to date, their production still mainly relies on extraction from plants. A five-step metabolic pathway was constructed in probiotic Lactococcus lactis NZ9000 for rapid, stable, and glycosylated anthocyanin biosynthesis using chalcone as a substrate. The genes were cloned from anthocyanin-rich blueberry: chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), anthocyanin synthase (ANS), and UDPG-flavonoid 3-O-glycosyltransferase (3GT). Using HR, the polysaccharide pellicle (PSP) segment of the cell wall polysaccharide synthesis (cwps) gene cluster from L. lactis NZ9000 was cloned into vector p15A-Cm-repDE. Then, CHI and F3H were placed sequentially under the control of NZProm 3 of this gene cluster in the vector, which was transformed into L. lactis NZ9000 to obtain Strain A. Furthermore, Strain B was constructed by placing F3H-DFR-ANS and 3GT under NZProm 2 and 3, respectively. Using LC-MS/MS analysis, several types of anthocyanins, including callistephin chloride, oenin chloride, malvidin O-hexoside, malvidin 3,5-diglucoside, and pelargonidin 3-O-malonyl-malonylhexoside, increased in the supernatant of the co-culture of Strains A and B compared to that of L. lactis NZ9000. This is the first time that a five-step metabolic pathway has been developed for anthocyanin biosynthesis in probiotic L. lactis NZ9000. This work lays the groundwork for novel anthocyanin production by a process involving the placement of several biosynthesis genes under the control of a gene cluster.

Seamless site-directed mutagenesis in complex cloned DNA sequences using the RedEx method

Seamless site-directed mutagenesis is an important technique for studying protein functions, tuning enzyme catalytic activities and modifying genetic elements in multiple rounds because it can insert, delete or substitute nucleotides, DNA segments or even entire genes at the target site without introducing any unwanted change. To facilitate seamless site-directed mutagenesis in large plasmids and bacterial artificial chromosomes (BACs) with repetitive sequences, we recently developed the RedEx strategy. Compared with previous methods, our approach achieves the recovery of correct recombinants with high accuracy by circumventing unwanted recombination between repetitive sequences. RedEx readily yields more than 80% accuracy in seamless DNA insertion and deletion in large multimodular polyketide synthase gene clusters, which are among the most difficult targets due to the large number of repetitive DNA sequences in modules encoding almost identical enzymes. Here we present the RedEx method by describing in detail the seamless site-directed mutagenesis in a BAC vector. Overall, the process includes three parts: (1) insertion of the RedEx cassette containing the desired mutation together with selection-counterselection markers flanked by unique restriction sites and 20-bp overlapping sequences into the target site by recombineering, (2) removal of the selection-counterselection markers in the BAC by restriction digestion and (3) circularization of the linear BAC by exonuclease-mediated in vitro DNA annealing. This protocol can be performed within 3 weeks and will enable researchers with DNA cloning experience to master seamless site-directed mutagenesis to accelerate their research.

Somatic and intergenerational G4C2 hexanucleotide repeat instability in a human C9orf72 knock-in mouse model

Expansion of a G4C2 repeat in the C9orf72 gene is associated with familial Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). To investigate the underlying mechanisms of repeat instability, which occurs both somatically and intergenerationally, we created a novel mouse model of familial ALS/FTD that harbors 96 copies of G4C2 repeats at a humanized C9orf72 locus. In mouse embryonic stem cells, we observed two modes of repeat expansion. First, we noted minor increases in repeat length per expansion event, which was dependent on a mismatch repair pathway protein Msh2. Second, we found major increases in repeat length per event when a DNA double- or single-strand break (DSB/SSB) was artificially introduced proximal to the repeats, and which was dependent on the homology-directed repair (HDR) pathway. In mice, the first mode primarily drove somatic repeat expansion. Major changes in repeat length, including expansion, were observed when SSB was introduced in one-cell embryos, or intergenerationally without DSB/SSB introduction if G4C2 repeats exceeded 400 copies, although spontaneous HDR-mediated expansion has yet to be identified. These findings provide a novel strategy to model repeat expansion in a non-human genome and offer insights into the mechanism behind C9orf72 G4C2 repeat instability.

Structural Basis for the Interaction of Redβ Single-Strand Annealing Protein with Escherichia coli Single-Stranded DNA-Binding Protein

Redβ is a protein from bacteriophage λ that binds to single-stranded DNA (ssDNA) to promote the annealing of complementary strands. Together with λ-exonuclease (λ-exo), Redβ is part of a two-component DNA recombination system involved in multiple aspects of genome maintenance. The proteins have been exploited in powerful methods for bacterial genome engineering in which Redβ can anneal an electroporated oligonucleotide to a complementary target site at the lagging strand of a replication fork. Successful annealing in vivo requires the interaction of Redβ with E. coli single-stranded DNA-binding protein (SSB), which coats the ssDNA at the lagging strand to coordinate access of numerous replication proteins. Previous mutational analysis revealed that the interaction between Redβ and SSB involves the C-terminal domain (CTD) of Redβ and the C-terminal tail of SSB (SSB-Ct), the site for binding of numerous host proteins. Here, we have determined the x-ray crystal structure of Redβ CTD in complex with a peptide corresponding to the last nine residues of SSB (MDFDDDIPF). Formation of the complex is predominantly mediated by hydrophobic interactions between two phenylalanine side chains of SSB (Phe-171 and Phe-177) and an apolar groove on the CTD, combined with electrostatic interactions between the C-terminal carboxylate of SSB and Lys-214 of the CTD. Mutation of any of these residues to alanine significantly disrupts the interaction of full-length Redβ and SSB proteins. Structural knowledge of this interaction will help to expand the utility of Redβ-mediated recombination to a wider range of bacterial hosts for applications in synthetic biology.

Multiplex CRISPR-Cas Genome Editing: Next-Generation Microbial Strain Engineering

Genome editing is a crucial technology for obtaining desired phenotypes in a variety of species, ranging from microbes to plants, animals, and humans. With the advent of CRISPR-Cas technology, it has become possible to edit the intended sequence by modifying the target recognition sequence in guide RNA (gRNA). By expressing multiple gRNAs simultaneously, it is possible to edit multiple targets at the same time, allowing for the simultaneous introduction of various functions into the cell. This can significantly reduce the time and cost of obtaining engineered microbial strains for specific traits. In this review, we investigate the resolution of multiplex genome editing and its application in engineering microorganisms, including bacteria and yeast. Furthermore, we examine how recent advancements in artificial intelligence technology could assist in microbial genome editing and engineering. Based on these insights, we present our perspectives on the future evolution and potential impact of multiplex genome editing technologies in the agriculture and food industry.

A unified mechanism for PARP inhibitor-induced PARP1 chromatin retention at DNA damage sites in living cells

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPis) not only suppress PARP1 catalytic activity but also prolong its association to damaged chromatin. Here, through live-cell imaging, we quantify the alterations in PARP1 dynamics and activity elicited by seven PARPis over a wide range of concentrations to deliver a unified mechanism of PARPi-induced PARP1 chromatin retention. We find that gross PARP1 retention at DNA damage sites is jointly governed by catalytic inhibition and allosteric trapping, albeit in a strictly independent manner-catalytic inhibition causes multiple unproductive binding-dissociation cycles of PARP1, while allosteric trapping prolongs the lesion-bound state of PARP1 to greatly increase overall retention. Importantly, stronger PARP1 retention produces greater temporal shifts in downstream DNA repair events and superior cytotoxicity, highlighting PARP1 retention, a complex but precisely quantifiable characteristic of PARPis, as a valuable biomarker for PARPi efficacy. Our approach can be promptly repurposed for interrogating the properties of DNA-repair-targeting compounds beyond PARPis.

Strategic combination of bacteriophages with highly susceptible cells for enhanced intestinal settlement and resistant cell killing

Avian pathogenic Escherichia coli (APEC) causes enormous economic losses and is a primary contributor to the emergence of multidrug resistance (MDR)-related problems in the poultry industry. Bacteriophage (phage) therapy has been successful in controlling MDR, but phage-resistant variants have rapidly emerged through the horizontal transmission of diverse phage defense systems carried on mobile genetic elements. Consequently, while multiple phage cocktails are recommended for phage therapy, there is a growing need to explore simpler and more cost-effective phage treatment alternatives. In this study, we characterized two novel O78-specific APEC phages, φWAO78-1 and φHAO78-1, in terms of their morphology, genome, physicochemical stability and growth kinetics. Additionally, we assessed the susceptibility of thirty-two O78 APEC strains to these phages. We analyzed the roles of highly susceptible cells in intestinal settlement and fecal shedding (susceptible cell-assisted intestinal settlement and shedding, SAIS) of phages in chickens via coinoculation with phages. Furthermore, we evaluated a new strategy, susceptible cell-assisted resistant cell killing (SARK), by comparing phage susceptibility between resistant cells alone and a mixture of resistant and highly susceptible cells in vitro. As expected, high proportions of O78 APEC strains had already acquired multiple phage defense systems, exhibiting considerable resistance to φWAO78-1 and φHAO78-1. Coinoculation of highly susceptible cells with phages prolonged phage shedding in feces, and the coexistence of susceptible cells markedly increased the phage susceptibility of resistant cells. Therefore, the SAIS and SARK strategies were demonstrated to be promising both in vivo and in vitro.

ORBIT for E. coli: kilobase-scale oligonucleotide recombineering at high throughput and high efficiency

Microbiology and synthetic biology depend on reverse genetic approaches to manipulate bacterial genomes; however, existing methods require molecular biology to generate genomic homology, suffer from low efficiency, and are not easily scaled to high throughput. To overcome these limitations, we developed a system for creating kilobase-scale genomic modifications that uses DNA oligonucleotides to direct the integration of a non-replicating plasmid. This method, Oligonucleotide Recombineering followed by Bxb-1 Integrase Targeting (ORBIT) was pioneered in Mycobacteria, and here we adapt and expand it for Escherichia coli. Our redesigned plasmid toolkit for oligonucleotide recombineering achieved significantly higher efficiency than λ Red double-stranded DNA recombineering and enabled precise, stable knockouts (≤134 kb) and integrations (≤11 kb) of various sizes. Additionally, we constructed multi-mutants in a single transformation, using orthogonal attachment sites. At high throughput, we used pools of targeting oligonucleotides to knock out nearly all known transcription factor and small RNA genes, yielding accurate, genome-wide, single mutant libraries. By counting genomic barcodes, we also show ORBIT libraries can scale to thousands of unique members (>30k). This work demonstrates that ORBIT for E. coli is a flexible reverse genetic system that facilitates rapid construction of complex strains and readily scales to create sophisticated mutant libraries.

Research Progress on the Assembly of Large DNA Fragments

Synthetic biology, a newly and rapidly developing interdisciplinary field, has demonstrated increasing potential for extensive applications in the wide areas of biomedicine, biofuels, and novel materials. DNA assembly is a key enabling technology of synthetic biology and a central point for realizing fully synthetic artificial life. While the assembly of small DNA fragments has been successfully commercialized, the assembly of large DNA fragments remains a challenge due to their high molecular weight and susceptibility to breakage. This article provides an overview of the development and current state of DNA assembly technology, with a focus on recent advancements in the assembly of large DNA fragments in Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae. In particular, the methods and challenges associated with the assembly of large DNA fragment in different hosts are highlighted. The advancements in DNA assembly have the potential to facilitate the construction of customized genomes, giving us the ability to modify cellular functions and even create artificial life. It is also contributing to our ability to understand, predict, and manipulate living organisms.

Rapid Rescue of Goose Astrovirus Genome via Red/ET Assembly

The host-specific infection of Avian Astrovirus (AAstVs) has posed significant challenges to the poultry industry, resulting in substantial economic losses. However, few reports exist on the functional consequences of genome diversity, cross-species infectivity and mechanisms governing virus replication of AAstVs, making it difficult to develop measures to control astrovirus transmission. Reverse genetics technique can be used to study the function of viruses at the molecular level, as well as investigating pathogenic mechanisms and guide vaccine development and disease treatment. Herein, the reverse genetics technique of goose astrovirus GAstV/JS2019 strain was developed based on use of a reconstructed vector including CMV promotor, hammerhead ribozyme (HamRz), hepatitis delta virus ribozyme (HdvRz), and SV40 tail, then the cloned viral genome fragments were connected using Red/ET recombineering. The recombinant rGAstV-JS2019 was readily rescued by transfected the infectious clone plasmid into LMH cells. Importantly, the rescued rGAstV/JS2019 exhibited similar growth kinetics comparable to those of the parental GAstV/JS2019 isolate in cultured cells. Our research results provide an alternative and more effective reverse genetic tool for a detailed understanding of viral replication, pathogenic mechanisms, and molecular mechanisms of evolution.

Standardized Iterative Genome Editing Method for Escherichia coli Based on CRISPR-Cas9

The introduction of complex biosynthetic pathways into the hosts' chromosomes is gaining attention with the development of synthetic biology. While CRISPR-Cas9 has been widely employed for gene knock-in, the process of multigene insertion remains cumbersome due to laborious and empirical gene cloning procedures. To address this, we devised a standardized iterative genome editing system for Escherichia coli, harnessing the power of CRISPR-Cas9 and MetClo assembly. This comprehensive toolkit comprises two fundamental elements based on the Golden Gate standard for modular assembly of sgRNA or CRISPR arrays and donor DNAs. We achieved a gene insertion efficiency of up to 100%, targeting a single locus. Expression of tracrRNA using a strong promoter enhances multiplex genomic insertion efficiency to 7.3%, compared with 0.76% when a native promoter is used. To demonstrate the robust capabilities of this genome editing toolbox, we successfully integrated 5-10 genes from the coenzyme B12 biosynthetic pathway ranging from 5.3 to 8 Kb in length into the chromosome of E. coli chassis cells, resulting in 14 antibiotic-free, plasmid-free producers. Following an extensive screening process involving genes from diverse sources, cistronic design modifications, and chromosome repositioning, we obtained a recombinant strain yielding 1.49 mg L-1 coenzyme B12, the highest known titer achieved by using E. coli as the producer. Illuminating its user-friendliness, this genome editing system is an exceedingly versatile tool for expediently integrating complex biosynthetic pathway genes into hosts' genomes, thus facilitating pathway optimization for chemical production.

Method for plasmid-based antibiotic-free fermentation

BACKGROUND

Antibiotic-based plasmid selection and maintenance is a core tool in molecular biology; however, while convenient, this strategy has numerous drawbacks for biological manufacturing. Overuse of antibiotics and antibiotic resistance genes (ARG) contributes to the development of antimicrobial resistance, which is a growing threat to modern medicine. Antibiotics themselves are costly and therefore often omitted in fermentations, leading to plasmid loss and a corresponding loss in product yield. Furthermore, constitutive expression of a plasmid-encoded antibiotic resistance gene imposes a significant metabolic burden on the cells. For many fermentation products (e.g., in nutrition and medicine), the use of antibiotic resistance genes is subject to strict regulations and should be avoided. We present a method for plasmid selection and maintenance with stringent selection pressure that is independent of antibiotics and ARG. Furthermore, it can be used without any restrictions regarding culture medium and temperature.

RESULTS

The developed method involves modification of a bacterial strain such that an essential gene is expressed genomically under the control of an inducible promoter. A copy of the same essential gene with the endogenous promoter is supplied on a plasmid for selection. In the absence of the inducer for the genomic copy of the essential gene, cells rely on expression of the plasmid-encoded gene copy, leading to tight selection for plasmid maintenance. Induction of the genomic copy of the essential gene enables the engineered strain to be propagated in the absence of a plasmid. Here, we describe the genetic setup and demonstrate long-term, tight selection for plasmid maintenance with a variety of different plasmids and E. coli strains.

CONCLUSIONS

This method facilitates plasmid-based fermentations by eliminating the need for antibiotic selection and improving plasmid maintenance.

Development of an Efficient and Seamless Genetic Manipulation Method for Xenorhabdus and Its Application for Enhancing the Production of Fabclavines

Xenorhabdus can produce numerous natural products, but their development has been hampered by the lack of a seamless genetic manipulation method. In this study, we compared several lethal genes and determined the sacB gene as the most effective counter-selection marker and then established a dual selection/counter-selection system by integrating neo and sacB genes into one cassette. This provides an efficient and seamless genetic manipulation method for Xenorhabdus. Using this method, DNA fragments ranging from 205 to 47,788 bp in length were seamlessly knocked out or replaced with impressively high positive rates of 80 to 100% in Xenorhabdus budapestensis XBD8. In addition, the method was successfully applied with good efficiency (45-100%) in Xenorhabdus nematophila CB6. To further validate the method, different constitutive promoters were used to replace the native fclC promoter in a batch experiment. The positivity rate remained consistently high, at 46.3%. In comparison to WT XBD8, the recombinant strain MX14 demonstrated a significant increase in the production of fabclavine 7 and fabclavine 8 by 4.97-fold and 3.22-fold, respectively, while the overall production of fabclavines was enhanced by 3.52-fold.

The Development of a CRISPR-FnCpf1 System for Large-Fragment Deletion and Multiplex Gene Editing in Acinetobacter baumannii

Acinetobacter baumannii is a low-GC-content Gram-negative opportunistic pathogen that poses a serious global public health threat. Convenient and rapid genetic manipulation is beneficial for elucidating its pathogenic mechanisms and developing novel therapeutic methods. In this study, we report a new CRISPR-FnCpf1-based two-plasmid system for versatile and precise genome editing in A. baumannii. After identification, this new system prefers to recognize the 5'-TTN-3' (N = A, T, C or G) and the 5'-CTV-3' (V = A, C or G) protospacer-adjacent motif (PAM) sequence and utilize the spacer with lengths ranging from 19 to 25 nt. In direct comparison with the existing CRISPR-Cas9 system, it exhibits approximately four times the targetable range in A. baumannii. Moreover, by employing a tandem dual crRNA expression cassette, the new system can perform large-fragment deletion and simultaneous multiple gene editing, which is difficult to achieve via CRISPR-Cas9. Therefore, the new system is valuable and can greatly expand the genome editing toolbox of A. baumannii.

The Diversity of Bacteriophages in Hot Springs

Bacteriophages are ubiquitous in all environments that support microbial life. This includes hot springs, which can range in temperatures between 40 and 98 °C and pH levels between 1 and 9. Bacteriophages that survive in the higher temperatures of hot springs are known as thermophages. Thermophages have developed distinct adaptations allowing for thermostability in these extreme environments, including increased G + C DNA percentages, reliance upon the pentose phosphate metabolic pathway to avoid oxidative stress, and a codon preference for those with a GNA sequence leading to increased hydrophobic interactions and disulfide bonds. In this review, we discuss the diversity of characterized thermophages in hot spring environments that span five viral families: Myoviridae, Siphoviridae, Tectiviridae, Sphaerolipoviridae, and Inoviridae. Potential industrial and medicinal applications of thermophages will also be addressed.

High-throughput ligand profile characterization in novel cell lines expressing seven heterologous insect olfactory receptors for the detection of volatile plant biomarkers

Agriculturally important crop plants emit a multitude of volatile organic compounds (VOCs), which are excellent indicators of their health status and their interactions with pathogens and pests. In this study, we have developed a novel cellular olfactory panel for detecting fungal pathogen-related VOCs we had identified in the field, as well as during controlled inoculations of several crop plants. The olfactory panel consists of seven stable HEK293 cell lines each expressing a functional Drosophila olfactory receptor as a biosensing element along with GCaMP6, a fluorescent calcium indicator protein. An automated 384-well microplate reader was used to characterize the olfactory receptor cell lines for their sensitivity to reference VOCs. Subsequently, we profiled a set of 66 VOCs on all cell lines, covering a concentration range from 1 to 100 μM. Results showed that 49 VOCs (74.2%) elicited a response in at least one olfactory receptor cell line. Some VOCs activated the cell lines even at nanomolar (ppb) concentrations. The interaction profiles obtained here will support the development of biosensors for agricultural applications. Additionally, the olfactory receptor proteins can be purified from these cell lines with sufficient yields for further processing, such as structure determination or integration with sensor devices.

GuaB3, an overlooked enzyme in cyanobacteria's toolbox that sheds light on IMP dehydrogenase evolution

IMP dehydrogenase and GMP reductase are enzymes from the same protein family with analogous structures and catalytic mechanisms that have gained attention because of their essential roles in nucleotide metabolism and as potential drug targets. This study focusses on GuaB3, a less-explored enzyme within this family. Phylogenetic analysis uncovers GuaB3's independent evolution from other members of the family and it predominantly occurs in Cyanobacteria. Within this group, GuaB3 functions as a unique IMP dehydrogenase, while its counterpart in Actinobacteria has a yet unknown function. Synechocystis sp. PCC6803 GuaB3 structures demonstrate differences in the active site compared to canonical IMP dehydrogenases, despite shared catalytic mechanisms. These findings highlight the essential role of GuaB3 in Cyanobacteria, provide insights into the diversity and evolution of the IMP dehydrogenase protein family, and reveal a distinctive characteristic in nucleotide metabolism, potentially aiding in combating harmful cyanobacterial blooms-a growing concern for humans and wildlife.

Construction and immune evaluation of the recombinant duck adenovirus type 3 delivering capsid protein VP1 of the type 1 duck hepatitis virus

Adenovirus serves as an excellent viral vector and is employed in vector vaccine research. Duck hepatitis A virus type 1 (DHAV1) and duck adenovirus type 3 (DAdV3) cause significant economic losses in the Chinese duck industry. In this study, we found an excellent exogenous gene insertion site in DAdV3 genome of CH-GD-12-2014 strain, within 3 intergenic regions (IGR). Subsequently, we generated a recombinant duck adenovirus named rDAdV3-VP1-188, which exhibits excellent replication characteristics and immunogenicity of DAdV3 and DHAV1. Animal experiments showed that rDAdV3-VP1-188 can provide 100% protection against the DAdV3 and 80% protection against DHAV1. These results showed that rDAdV3-VP1-188 could induce protection against DAdV3 and DHAV1 in ducks, thus indicating the feasibility of DAdV3 as a vector for the development of avian vector vaccines. These insights contribute to the further development of DAdV3 vectors and other adenovirus vectors.

Single-cell RNA sequencing unravels the transcriptional network underlying zebrafish retina regeneration

In the lesioned zebrafish retina, Müller glia produce multipotent retinal progenitors that generate all retinal neurons, replacing lost cell types. To study the molecular mechanisms linking Müller glia reactivity to progenitor production and neuronal differentiation, we used single-cell RNA sequencing of Müller glia, progenitors and regenerated progeny from uninjured and light-lesioned retinae. We discover an injury-induced Müller glia differentiation trajectory that leads into a cell population with a hybrid identity expressing marker genes of Müller glia and progenitors. A glial self-renewal and a neurogenic trajectory depart from the hybrid cell population. We further observe that neurogenic progenitors progressively differentiate to generate retinal ganglion cells first and bipolar cells last, similar to the events observed during retinal development. Our work provides a comprehensive description of Müller glia and progenitor transcriptional changes and fate decisions in the regenerating retina, which are key to tailor cell differentiation and replacement therapies for retinal dystrophies in humans.

DNA Recombineering Applications

The ability to manipulate the bacterial genome is an obligatory premise for the study of gene function and regulation in bacterial cells. The λ red recombineering technique allows modification of chromosomal sequences with base-pair precision without the need of intermediate molecular cloning steps. Initially conceived to construct insertion mutants, the technique lends itself to a wide variety of applications including the creation of point mutants, seamless deletions, reporter, and epitope tag fusions and chromosomal rearrangements. Here, we introduce some of the most common implementations of the method.

Precise genome engineering in Pseudomonas using phage-encoded homologous recombination and the Cascade-Cas3 system

A lack of generic and effective genetic manipulation methods for Pseudomonas has restricted fundamental research and utilization of this genus for biotechnology applications. Phage-encoded homologous recombination (PEHR) is an efficient tool for bacterial genome engineering. This PEHR system is based on a lambda Red-like operon (BAS) from Pseudomonas aeruginosa phage Ab31 and a Rac bacteriophage RecET-like operon (Rec-TEPsy) from P. syringae pv. syringae B728a and also contains exogenous elements, including the RecBCD inhibitor (Redγ or Pluγ) or single-stranded DNA-binding protein (SSB), that were added to enhance the PEHR recombineering efficiency. To solve the problem of false positives in Pseudomonas editing with the PEHR system, the processive enzyme Cas3 with a minimal Type I-C Cascade-based system was combined with PEHR. This protocol describes the utilization of a Pseudomonas-specific PEHR-Cas3 system that was designed to universally and proficiently modify the genomes of Pseudomonas species. The pipeline uses standardized cassettes combined with the concerted use of SacB counterselection and Cre site-specific recombinase for markerless or seamless genome modification, in association with vectors that possess the selectively replicating template R6K to minimize recombineering background. Compared with the traditional allelic exchange editing method, the PEHR-Cas3 system does not need to construct suicide plasmids carrying long homologous arms, thus simplifying the experimental procedure and shortening the traceless editing period. Compared with general editing systems based on phage recombinases, the PEHR-Cas3 system can effectively improve the screening efficiency of mutants using the cutting ability of Cas3 protein. The entire procedure requires ~12 days.

Construction of a bacteriophage-derived recombinase system in Bacillus licheniformis for gene deletion

Bacillus licheniformis and its related strains have found extensive applications in diverse industries, agriculture, and medicine. However, the current breeding methods for this strain primarily rely on natural screening and traditional mutagenesis. The limited availability of efficient genetic engineering tools, particularly recombination techniques, has hindered further advancements in its applications. In this study, we conducted a comprehensive investigation to identify and characterize a recombinase, RecT, derived from a Bacillus phage. Remarkably, the recombinase exhibited a 105-fold enhancement in the recombination efficiency of the strain. To facilitate genome editing, we developed a system based on the conditional expression of RecT using a rhamnose-inducible promoter (Prha). The efficacy of this system was evaluated by deleting the amyL gene, which encodes an α-amylase. Our findings revealed that the induction time and concentration of rhamnose, along with the generation time of the strain, significantly influenced the editing efficiency. Optimal conditions for genome editing were determined as follows: the wild-type strain was initially transformed with the genome editing plasmid, followed by cultivation and induction with 1.5% rhamnose for 8 h. Subsequently, the strain was further cultured for an additional 24 h, equivalent to approximately three generations. Consequently, the recombination efficiency reached an impressive 16.67%. This study represents a significant advancement in enhancing the recombination efficiency of B. licheniformis through the utilization of a RecT-based recombination system. Moreover, it provides a highly effective genome editing tool for genetic engineering applications in this strain.

Review of knockout technology approaches in bacterial drug resistance research

Gene knockout is a widely used method in biology for investigating gene function. Several technologies are available for gene knockout, including zinc-finger nuclease technology (ZFN), suicide plasmid vector systems, transcription activator-like effector protein nuclease technology (TALEN), Red homologous recombination technology, CRISPR/Cas, and others. Of these, Red homologous recombination technology, CRISPR/Cas9 technology, and suicide plasmid vector systems have been the most extensively used for knocking out bacterial drug resistance genes. These three technologies have been shown to yield significant results in researching bacterial gene functions in numerous studies. This study provides an overview of current gene knockout methods that are effective for genetic drug resistance testing in bacteria. The study aims to serve as a reference for selecting appropriate techniques.

Markerless bacterial artificial chromosome manipulation method by red proteins of phage λ mediated homologous recombination utilizing fluorescent proteins for both positive and counter selection

Manipulating viral genomes is an essential technique in reverse genetics and recombinant vaccine development. A strategy for manipulating large viral genomes involves introducing their entire genome into bacterial artificial chromosomes and employing Escherichia coli genetic tools. For sequence manipulation on bacterial artificial chromosomes (bacterial artificial chromosomes recombineering), a well-established method that relies on the Escherichia coli strain GS1783, and the template plasmid, pEPKan-S, is often used. This method, known as markerless DNA manipulation, allows for the generation of a recombinant bacterial artificial chromosome that does not retain the selection markers used during recombination. Although this method is highly innovative, there remains room for improvement as the plasmid is currently only available for positive selection. Additionally, differentiating true recombinants from false negatives often proves time-consuming. Consequently, an improved method for bacterial artificial chromosomes recombineering, which utilizes fluorescent proteins, has been developed. This method's core comprises three plasmids containing the I-SceI recognition site, antibiotic resistance genes (ampicillin, kanamycin, and zeocin), and fluorescent genes (YPet, mOrange, and mScarlet). The success or failure of Red recombination can be confirmed via fluorescent signals. To validate this method, the Lassa virus genes were introduced into the bacterial artificial chromosomes, containing the entire genome of the vaccinia virus strain LC16m8. Consequently, the expression of fluorescent protein genes contributed to positive selection, such as blue-white screening and counter-selection during the first and second Red recombination.

Quick and affordable DNA cloning by reconstitution of Seamless Ligation Cloning Extract using defined factors

The cloning of DNA fragments to plasmid vectors is at the heart of molecular biology. Recent developments have led to various methods utilizing homologous recombination of homology arms. Among them, Seamless Ligation Cloning Extract (SLiCE) is an affordable alternative solution that uses simple Escherichia coli lysates. However, the underlying molecular mechanisms remain unclear and the reconstitution of the extract by defined factors has not yet been reported. We herein show that the key factor in SLiCE is Exonuclease III (ExoIII), a double-strand (ds) DNA-dependent 3'-5' exonuclease, encoded by XthA. SLiCE prepared from the xthAΔ strain is devoid of recombination activity, whereas purified ExoIII alone is sufficient to assemble two blunt-ended dsDNA fragments with homology arms. In contrast to SLiCE, ExoIII is unable to digest (or assemble) fragments with 3' protruding ends; however, the addition of single-strand DNA-targeting Exonuclease T overcomes this issue. Through the combination of commercially available enzymes under optimized conditions, we achieved the efficient, reproducible, and affordable cocktail, "XE cocktail," for seamless DNA cloning. By reducing the cost and time required for DNA cloning, researchers will devote more resources to advanced studies and the careful validation of their own findings.

Development of a Streptomyces-based system for facile thioholgamide library generation and analysis

Derivatizing natural products (NPs) is essential in structure-activity relationship (SAR) studies, compound optimization, and drug development. Ribosomally synthesized and post-translationally modified peptides (RiPPs) represent one of the major classes of natural products. Thioholgamide represents thioamitide - a recently emerged family of RiPPs with unique structures and great potential in anticancer drug development. Although the method for generating the RiPP library by codon substitutions in the precursor peptide gene is straightforward, the techniques to perform RiPP derivatization in Actinobacteria remain limited and time-consuming. Here, we report a facile system for producing a library of randomized thioholgamide derivatives utilizing an optimized Streptomyces host. This technique enabled us to access all possible amino acid substitutions of the thioholgamide molecule, one position at a time. Out of 152 potential derivatives, 85 were successfully detected, revealing the impact of amino acid substitutions on thioholgamide post-translational modifications (PTMs). Moreover, new PTMs were observed among thioholgamide derivatives: thiazoline heterocycles, which have not yet been reported for thioamitides, and S-methylmethionine, which is very rare in nature. The obtained library was subsequently used for thioholgamide SAR studies and stability assays.

Bacterial synthetic biology: tools for novel drug discovery

INTRODUCTION

Bacterial synthetic biology has provided powerful tools to revolutionize the drug discovery process. These tools can be harnessed to generate bacterial novel pharmaceutical compounds with enhanced bioactivity and selectivity or to create genetically modified microorganisms as living drugs.

AREAS COVERED

This review provides a current overview of the state-of-the-art in bacterial synthetic biology tools for novel drug discovery. The authors discuss the application of these tools including bioinformatic tools, CRISPR tools, engineered bacterial transcriptional regulators, and synthetic biosensors for novel drug discovery. Additionally, the authors present the recent progress on reprogramming bacteriophages as living drugs to fight against antibiotic-resistant pathogens.

EXPERT OPINION

The field of using bacterial synthetic biology tools for drug discovery is rapidly advancing. However, challenges remain in developing reliable and robust methods to engineer bacteria. Further advancements in synthetic biology hold promise to speed up drug discovery, facilitating the development of novel therapeutics against various diseases.

CRISPR-Associated Transposase for Targeted Mutagenesis in Diverse Proteobacteria

Genome editing tools, through the disruption of an organism's native genetic material or the introduction of non-native DNA, facilitate functional investigations to link genotypes to phenotypes. Transposons have been instrumental genetic tools in microbiology, enabling genome-wide, randomized disruption of genes and insertions of new genetic elements. Due to this randomness, identifying and isolating particular transposon mutants (i.e., those with modifications at a genetic locus of interest) can be laborious, often requiring one to sift through hundreds or thousands of mutants. Programmable, site-specific targeting of transposons became possible with recently described CRISPR-associated transposase (CASTs) systems, allowing the streamlined recovery of desired mutants in a single step. Like other CRISPR-derived systems, CASTs can be programmed by guide-RNA that is transcribed from short DNA sequence(s). Here, we describe a CAST system and demonstrate its function in bacteria from three classes of Proteobacteria. A dual plasmid strategy is demonstrated: (i) CAST genes are expressed from a broad-host-range replicative plasmid and (ii) guide-RNA and transposon are encoded on a high-copy, suicidal pUC plasmid. Using our CAST system, single-gene disruptions were performed with on-target efficiencies approaching 100% in Beta- and Gammaproteobacteria (Burkholderia thailandensis and Pseudomonas putida, respectively). We also report a peak efficiency of 45% in the Alphaproteobacterium Agrobacterium fabrum. In B. thailandensis, we performed simultaneous co-integration of transposons at two different target sites, demonstrating CAST's utility in multilocus strategies. The CAST system is also capable of high-efficiency large transposon insertion totaling over 11 kbp in all three bacteria tested. Lastly, the dual plasmid system allowed for iterative transposon mutagenesis in all three bacteria without loss of efficiency. Given these iterative capabilities and large payload capacity, this system will be helpful for genome engineering experiments across several fields of research.

Applications of Programmable Endonucleases in Sequence- and Ligation-Independent Seamless DNA Assembly

Programmable endonucleases, such as Cas (Clustered Regularly-Interspaced Short Repeats-associated proteins) and prokaryotic Argonaute (pAgo), depend on base pairing of the target DNA with the guide RNA or DNA to cleave DNA strands. Therefore, they are capable of recognizing and cleaving DNA sequences at virtually any arbitrary site. The present review focuses on the commonly used in vivo and in vitro recombination-based gene cloning methods and the application of programmable endonucleases in these sequence- and ligation-independent DNA assembly methods. The advantages and shortcomings of the programmable endonucleases utilized as tools for gene cloning are also discussed in this review.

Cross-species microbial genome transfer: a Review

Synthetic biology combines the disciplines of biology, chemistry, information science, and engineering, and has multiple applications in biomedicine, bioenergy, environmental studies, and other fields. Synthetic genomics is an important area of synthetic biology, and mainly includes genome design, synthesis, assembly, and transfer. Genome transfer technology has played an enormous role in the development of synthetic genomics, allowing the transfer of natural or synthetic genomes into cellular environments where the genome can be easily modified. A more comprehensive understanding of genome transfer technology can help to extend its applications to other microorganisms. Here, we summarize the three host platforms for microbial genome transfer, review the recent advances that have been made in genome transfer technology, and discuss the obstacles and prospects for the development of genome transfer.

Behavioral and Transcriptome Profiling of Heterozygous Rab10 Knock-Out Mice

A central question in the field of aging research is to identify the cellular and molecular basis of neuroresilience. One potential candidate is the small GTPase, Rab10. Here, we used Rab10+/- mice to investigate the molecular mechanisms underlying Rab10-mediated neuroresilience. Brain expression analysis of 880 genes involved in neurodegeneration showed that Rab10+/- mice have increased activation of pathways associated with neuronal metabolism, structural integrity, neurotransmission, and neuroplasticity compared with their Rab10+/+ littermates. Lower activation was observed for pathways involved in neuroinflammation and aging. We identified and validated several differentially expressed genes (DEGs), including Stx2, Stx1b, Vegfa, and Lrrc25 (downregulated) and Prkaa2, Syt4, and Grin2d (upregulated). Behavioral testing showed that Rab10+/- mice perform better in a hippocampal-dependent spatial task (object in place test), while their performance in a classical conditioning task (trace eyeblink classical conditioning, TECC) was significantly impaired. Therefore, our findings indicate that Rab10 differentially controls the brain circuitry of hippocampal-dependent spatial memory and higher-order behavior that requires intact cortex-hippocampal circuitry. Transcriptome and biochemical characterization of these mice suggest that glutamate ionotropic receptor NMDA type subunit 2D (GRIN2D or GluN2D) is affected by Rab10 signaling. Further work is needed to evaluate whether GRIN2D mediates the behavioral phenotypes of the Rab10+/- mice. We conclude that Rab10+/- mice described here can be a valuable tool to study the mechanisms of resilience in Alzheimer's disease (AD) model mice and to identify novel therapeutical targets to prevent cognitive decline associated with normal and pathologic aging.

The genome editing revolution

A series of spectacular scientific discoveries and technological advances in the second half of the 20th century have provided the basis for the ongoing genome editing revolution. The elucidation of structural and functional features of DNA and RNA was followed by pioneering studies on genome editing: Molecular biotechnology was born. Since then, four decades followed during which progress of scientific insights and technological methods continued at an overwhelming pace. Fundamental insights into microbial host-virus interactions led to the development of tools for genome editing using restriction enzymes or the revolutionary CRISPR-Cas technology. In this review, we provide a historical overview of milestones that led to the genome editing revolution and speculate about future trends in biotechnology.

Metabolic engineering for sustainability and health

Bio-based production of chemicals and materials has attracted much attention due to the urgent need to establish sustainability and enhance human health. Metabolic engineering (ME) allows purposeful modification of cellular metabolic, regulatory, and signaling networks to achieve enhanced production of desired chemicals and degradation of environmentally harmful chemicals. ME has significantly progressed over the past 30 years through further integration of the strategies of synthetic biology, systems biology, evolutionary engineering, and data science aided by artificial intelligence. Here we review the field of ME from its emergence to the current state-of-the-art, highlighting its contribution to sustainable production of chemicals, health, and the environment through representative examples. Future challenges of ME and perspectives are also discussed.

Bacterial-Artificial-Chromosome-Based Genome Editing Methods and the Applications in Herpesvirus Research

Herpesviruses are major pathogens that infect humans and animals. Manipulating the large genome is critical for exploring the function of specific genes and studying the pathogenesis of herpesviruses and developing novel anti-viral vaccines and therapeutics. Bacterial artificial chromosome (BAC) technology significantly advanced the capacity of herpesviruses researchers to manipulate the virus genomes. In the past years, advancements in BAC-based genome manipulating and screening strategies of recombinant BACs have been achieved, which has promoted the study of the herpes virus. This review summarizes the advances in BAC-based gene editing technology and selection strategies. The merits and drawbacks of BAC-based herpesvirus genome editing methods and the application of BAC-based genome manipulation in viral research are also discussed. This review provides references relevant for researchers in selecting gene editing methods in herpes virus research. Despite the achievements in the genome manipulation of the herpes viruses, the efficiency of BAC-based genome manipulation is still not satisfactory. This review also highlights the need for developing more efficient genome-manipulating methods for herpes viruses.

Recombineering: Genetic Engineering in Escherichia coli Using Homologous Recombination

The bacterial chromosome and bacterial plasmids can be engineered in vivo by homologous recombination using either PCR products or synthetic double-stranded DNA (dsDNA) or single-stranded DNA as substrates. Multiple linear dsDNA molecules can be assembled into an intact plasmid. The technology of recombineering is possible because bacteriophage-encoded recombination proteins efficiently recombine sequences with homologies as short as 35 to 50 bases. Recombineering allows DNA sequences to be inserted or deleted without regard to the location of restriction sites and can also be used in combination with CRISPR/Cas targeting systems. © 2023 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: Making electrocompetent cells and transforming with linear DNA Support Protocol 1: Selection/counter-selections for genome engineering Support Protocol 2: Creating and screening for oligo recombinants by PCR Support Protocol 3: Other methods of screening for unselected recombinants Support Protocol 4: Curing recombineering plasmids containing a temperature-sensitive replication function Support Protocol 5: Removal of the prophage by recombineering Alternate Protocol 1: Using CRISPR/Cas9 as a counter-selection following recombineering Alternate Protocol 2: Assembly of linear dsDNA fragments into functional plasmids Alternate Protocol 3: Retrieval of alleles onto a plasmid by gap repair Alternate Protocol 4: Modifying multicopy plasmids with recombineering Support Protocol 6: Screening for unselected plasmid recombinants Alternate Protocol 5: Recombineering with an intact λ prophage Alternate Protocol 6: Targeting an infecting λ phage with the defective prophage strains.

Application of Cloning-Free Genome Engineering to Escherichia coli

The propagation of foreign DNA in Escherichia coli is central to molecular biology. Recent advances have dramatically expanded the ability to engineer (bacterial) cells; however, most of these techniques remain time-consuming. The aim of the present work was to explore the possibility to use the cloning-free genome editing (CFGE) approach, proposed by Döhlemann and coworkers (2016), for E. coli genetics, and to deepen the knowledge about the homologous recombination mechanism. The E. coli auxotrophic mutant strains FB182 (hisF892) and FB181 (hisI903) were transformed with the circularized wild-type E. coli (i) hisF gene and hisF gene fragments of decreasing length, and (ii) hisIE gene, respectively. His⁺ clones were selected based on their ability to grow in the absence of histidine, and their hisF/hisIE gene sequences were characterized. CFGE method allowed the recombination of wild-type his genes (or fragments of them) within the mutated chromosomal copy, with a different recombination frequency based on the fragment length, and the generation of clones with a variable number of in tandem his genes copies. Data obtained pave the way to further evolutionary studies concerning the homologous recombination mechanism and the fate of in tandem duplicated genes.

Establishing genetic manipulation for novel strains of human gut bacteria

Recent years have seen the development of high-accuracy and high-throughput genetic manipulation techniques, which have greatly improved our understanding of genetically tractable microbes. However, challenges remain in establishing genetic manipulation techniques in novel organisms, owing largely to exogenous DNA defence mechanisms, lack of selectable markers, lack of efficient methods to introduce exogenous DNA and an inability of genetic vectors to replicate in their new host. In this review, we describe some of the techniques that are available for genetic manipulation of novel microorganisms. While many reviews exist that focus on the final step in genetic manipulation, the editing of recipient DNA, we particularly focus on the first step in this process, the transfer of exogenous DNA into a strain of interest. Examples illustrating the use of these techniques are provided for a selection of human gut bacteria in which genetic tractability has been established, such as Bifidobacterium, Bacteroides and Roseburia. Ultimately, this review aims to provide an information source for researchers interested in developing genetic manipulation techniques for novel bacterial strains, particularly those of the human gut microbiota.

Efficiency of CRISPR-Cas9 genetic engineering in Escherichia coli BL21 is impaired by lack of Lon protease

The efficiency with which E.coli BL21 can be modified using CRISPR-Cas9 genetic engineering is several orders of magnitude lower than that of E. coli W3110. We show that the lack of Lon protease is responsible, and demonstrate that restoration of the Lon protease or knock-out of sulA improves CRISPR-Cas9 engineering efficiency of BL21 to levels comparable to E. coli W3110.

Application of an Escherichia coli triple reporter strain for at-line monitoring of single-cell physiology during L-phenylalanine production

Biotechnological production processes are sustainable approaches for the production of biobased components such as amino acids for food and feed industry. Scale-up from ideal lab-scale bioreactors to large-scale processes is often accompanied by loss in productivity. This may be related to population heterogeneities of cells originating from isogenic cultures that arise due to dynamic non-ideal conditions in the bioreactor. To better understand this phenomenon, deeper insights into single-cell physiologies in bioprocesses are mandatory before scale-up. Here, a triple reporter strain (3RP) was developed by chromosomally integrating the fluorescent proteins mEmerald, CyOFP1, and mTagBFP2 into the L-phenylalanine producing Escherichia coli strain FUS4 (pF81kan) to allow monitoring of growth, oxygen availability, and general stress response of the single cells. Functionality of the 3RP was confirmed in well-mixed lab-scale fed-batch processes with glycerol as carbon source in comparison to the strain without fluorescent proteins, leading to no difference in process performance. Fluorescence levels could successfully reflect the course of related process state variables, revealed population heterogeneities during the transition between different process phases and potentially subpopulations that exhibit superior process performance. Furthermore, indications were found for noise in gene expression as regulation strategy against environmental perturbation.

Structure of a RecT/Redβ family recombinase in complex with a duplex intermediate of DNA annealing

Some bacteriophage encode a recombinase that catalyzes single-stranded DNA annealing (SSA). These proteins are apparently related to RAD52, the primary human SSA protein. The best studied protein, Redβ from bacteriophage λ, binds weakly to ssDNA, not at all to dsDNA, but tightly to a duplex intermediate of annealing formed when two complementary DNA strands are added to the protein sequentially. We used single particle cryo-electron microscopy (cryo-EM) to determine a 3.4 Å structure of a Redβ homolog from a prophage of Listeria innocua in complex with two complementary 83mer oligonucleotides. The structure reveals a helical protein filament bound to a DNA duplex that is highly extended and unwound. Native mass spectrometry confirms that the complex seen by cryo-EM is the predominant species in solution. The protein shares a common core fold with RAD52 and a similar mode of ssDNA-binding. These data provide insights into the mechanism of protein-catalyzed SSA.

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.

Coupling CRISPR/Cas9 and Lambda Red Recombineering System for Genome Editing of Salmonella Gallinarum and the Effect of ssaU Knock-Out Mutant on the Virulence of Bacteria

The poultry industry in developing countries still faces a significant threat from fowl typhoid, a disease caused by Salmonella Gallinarum that has been well contained in more economically developed countries. In addition to the virulence exhibited by large virulence plasmid (85 kb), Salmonella Pathogenicity Island 2 in S. Gallinarum plays a key role in mediating disease through its type III secretion systems (TTSS). The TTSS secrete effector protein across the Salmonella containing vacuoles and mediate the internalization of bacteria by modulating vesicular passage. In this study, candidate virulent ssaU gene (~1 kb) encoding type III secretion system was successfully deleted from indigenously isolated S. Gallinarum genome through homology-directed repair using CRISPR/Cas9 and lambda recombination systems. CRISPR/Cas9-based genome editing of poultry-derived Salmonella Gallinarum has not been previously reported, which might be linked to a lack of efficiency in its genetic tools. This is the first study which demonstrates a complete CRISPR/Cas9-based gene deletion from this bacterial genome. More importantly, a poultry experimental model was employed to assess the virulence potential of this mutant strain (ΔssaU_SG18) which was unable to produce any mortality in the experimentally challenged birds as compared to the wild type strain. No effect on weight gain was observed whereas bacteria were unable to colonize the intestine and liver in our challenge model. This in vivo loss of virulence in mutant strain provides an excellent functionality of this system to be useful in live vaccine development against this resistant and patho genic bacteria.

Engineering of Burkholderia thailandensis strain E264 serves as a chassis for expression of complex specialized metabolites

Heterologous expression is an indispensable approach to exploiting natural products from phylogenetically diverse microbial communities. In this study, we constructed a heterologous expression system based on strain Burkholderia thailandensis E264 by deleting efflux pump genes and screening constitutive strong promoters. The biosynthetic gene cluster (BGC) of disorazol from Sorangium cellulosum So ce12 was expressed successfully with this host, and the yield of its product, disorazol F2, rather than A1, was improved to 38.3 mg/L by promoter substitution and insertion. In addition to the disorazol gene cluster, the BGC of rhizoxin from Burkholderia rhizoxinica was also expressed efficiently, whereas no specific peak was detected when shuangdaolide BGC from Streptomyces sp. B59 was transformed into the host. This system provides another option to explore natural products from different phylogenetic taxa.

HEHR: Homing Endonuclease-Mediated Homologous Recombination for Efficient Adenovirus Genome Engineering

Adenoviruses are non-enveloped linear double-stranded DNA viruses with over 100 types in humans. Adenovirus vectors have gained tremendous attention as gene delivery vehicles, as vaccine vectors and as oncolytic viruses. Although various methods have been used to generate adenoviral vectors, the vector-producing process remains technically challenging regarding efficacious genome modification. Based on our previously reported adenoviral genome modification streamline via linear–circular homologous recombination, we further develop an HEHR (combining Homing Endonucleases and Homologous Recombination) method to engineer adenoviral genomes more efficiently. I-PpoI, a rare endonuclease encoded by a group I intron, was introduced into the previously described ccdB counter-selection marker. We found that the I-PpoI pre-treatment of counter-selection containing parental plasmid increased the homologous recombination efficiency up to 100%. The flanking of the counter-selection marker with either single or double I-PpoI sites showed enhanced efficacy. In addition, we constructed a third counter-selection marker flanked by an alternative restriction enzyme: AbsI, which could be applied in case the I-PpoI site already existed in the transgene cassette that was previously inserted in the adenovirus genome. Together, HEHR can be applied for seamless sequence replacements, deletions and insertions. The advantages of HEHR in seamless mutagenesis will facilitate rational design of adenoviral vectors for diverse purposes.

Mutagenesis and Resistance Development of Bacteria Challenged by Silver Nanoparticles

Because of their extremely broad spectrum and strong biocidal power, nanoparticles of metals, especially silver (AgNPs), have been widely applied as effective antimicrobial agents against bacteria, fungi, and so on. However, the mutagenic effects of AgNPs and resistance mechanisms of target cells remain controversial. In this study, we discover that AgNPs do not speed up resistance mutation generation by accelerating genome-wide mutation rate of the target bacterium Escherichia coli. AgNPs-treated bacteria also show decreased expression in quorum sensing (QS), one of the major mechanisms leading to population-level drug resistance in microbes. Nonetheless, these nanomaterials are not immune to resistance development by bacteria. Gene expression analysis, experimental evolution in response to sublethal or bactericidal AgNPs treatments, and gene editing reveal that bacteria acquire resistance mainly through two-component regulatory systems, especially those involved in metal detoxification, osmoregulation, and energy metabolism. Although these findings imply low mutagenic risks of nanomaterial-based antimicrobial agents, they also highlight the capacity for bacteria to evolve resistance.

Heterologous Assembly of the Type VI Secretion System Empowers Laboratory Escherichia coli with Antimicrobial and Cell Penetration Capabilities

The synthetic biology toolbox has amassed a vast number of diverse functional modules, but protein translocation modules for cell penetration and cytosol-to-cytosol delivery remain relatively scarce. The type VI secretion system (T6SS), commonly found in many Gram-negative pathogens, functions as a contractile device to translocate protein toxins to prokaryotic and eukaryotic cells. Here, we have assembled the T6SS of Aeromonas dhakensis, an opportunistic waterborne pathogen, in the common laboratory strain Escherichia coli BL21(DE3). We constructed a series of plasmids (pT6S) carrying the T6SS structural and effector genes under native or tetracycline-inducible promoters, the latter for controlled expression. Using fluorescence microscopy and biochemical analyses, we demonstrate a functional T6SS in E. coli capable of secreting proteins directly into the cytosol of neighboring bacteria and outcompeting a number of drug-resistant pathogens. The heterologous assembly of T6SS not only confers the lab workhorse E. coli with the cytosol-to-cytosol protein delivery capability but also demonstrates the potential for harnessing the T6SS of various pathogens for general protein delivery and antibacterial applications. IMPORTANCE The T6SS is a powerful and versatile protein delivery system. However, the complexity of its macromolecular structure and gene regulation makes it not a trivial task to reconstitute the T6SSs of pathogens in a nonpathogenic host. In this study, we have assembled an inducible T6SS in E. coli BL21(DE3) and demonstrated its functions in protein delivery and antimicrobial activities. The engineered T6SS empowers E. coli to deliver protein cargos into a wide range of prokaryotic and eukaryotic cells.

SURE editing: combining oligo-recombineering and programmable insertion/deletion of selection markers to efficiently edit the Mycoplasma pneumoniae genome

The development of advanced genetic tools is boosting microbial engineering which can potentially tackle wide-ranging challenges currently faced by our society. Here we present SURE editing, a multi-recombinase engineering rationale combining oligonucleotide recombineering with the selective capacity of antibiotic resistance via transient insertion of selector plasmids. We test this method in Mycoplasma pneumoniae, a bacterium with a very inefficient native recombination machinery. Using SURE editing, we can seamlessly generate, in a single step, a wide variety of genome modifications at high efficiencies, including the largest possible deletion of this genome (30 Kb) and the targeted complementation of essential genes in the deletion of a region of interest. Additional steps can be taken to remove the selector plasmid from the edited area, to obtain markerless or even scarless edits. Of note, SURE editing is compatible with different site-specific recombinases for mediating transient plasmid integration. This battery of selector plasmids can be used to select different edits, regardless of the target sequence, which significantly reduces the cloning load associated to genome engineering projects. Given the proven functionality in several microorganisms of the machinery behind the SURE editing logic, this method is likely to represent a valuable advance for the synthetic biology field.