Functional characterization and application of a tightly regulated MekR/P mekA expression system in Escherichia coli and Pseudomonas putida

CRISPR/Cas9-assisted ssDNA recombineering for site-directed mutagenesis and saturation mutagenesis of plasmid-encoded genes

Site-directed and saturation mutagenesis are critical DNA methodologies for studying protein structure and function. For plasmid-based gene mutation, PCR and overlap-extension PCR involve tedious cloning steps. When the plasmid size is large, PCR yield may be too low for cloning; and for saturation mutagenesis of a single codon, one experiment may not enough to generate all twenty coding variants. Oligo-mediated recombineering sidesteps the complicated cloning process by homologous recombination between a mutagenic oligo and its target site. However, the low recombineering efficiency and inability to select for the recombinant makes it necessary to screen a large number of clones. Herein, we describe two plasmid-based mutagenic strategies: CRISPR/Cas9-assisted ssDNA recombineering for site-directed mutagenesis (CRM) and saturation mutagenesis (CRSM). CRM and CRSM involve co-electroporation of target plasmid, sgRNA expression plasmid and mutagenic oligonucleotide into Escherichia coli cells with induced expression of λ-Red recombinase and Cas9, followed by plasmid extraction and characterization. We established CRM and CRSM via ampicillin resistance gene repair and mutagenesis of N-acetyl‑D‑neuraminic acid aldolase. The mutational efficiency was between 80 and 100% and all twenty amino acid coding variants were obtained at a target site via a single CRSM strategy. CRM and CRSM have the potential to be general plasmid-based gene mutagenesis tools.

Development of dual-inducible duet-expression vectors for tunable gene expression control and CRISPR interference-based gene repression in Pseudomonas putida KT2440

The development of P. putida as an industrial host requires a sophisticated molecular toolbox for strain improvement, including vectors for gene expression and repression. To augment existing expression plasmids for metabolic engineering, we developed a series of dual-inducible duet-expression vectors for P. putida KT2440. A number of inducible promoters (Plac , Ptac , PtetR/tetA and Pbad ) were used in different combinations to differentially regulate the expression of individual genes. Protein expression was evaluated by measuring the fluorescence of reporter proteins (GFP and RFP). Our experiments demonstrated the use of compatible plasmids, a useful approach to coexpress multiple genes in P. putida KT2440. These duet vectors were modified to generate a fully inducible CRISPR interference system using two catalytically inactive Cas9 variants from S. pasteurianus (dCas9) and S. pyogenes (spdCas9). The utility of developed CRISPRi system(s) was demonstrated by repressing the expression of nine conditionally essential genes, resulting in growth impairment and prolonged lag phase for P. putida KT2440 growth on glucose. Furthermore, the system was shown to be tightly regulated, tunable and to provide a simple way to identify essential genes with an observable phenotype.

A plasmid toolbox for controlled gene expression across the Proteobacteria

Controlled gene expression is fundamental for the study of gene function and our ability to engineer bacteria. However, there is currently no easy-to-use genetics toolbox that enables controlled gene expression in a wide range of diverse species. To facilitate the development of genetics systems in a fast, easy, and standardized manner, we constructed and tested a plasmid assembly toolbox that will enable the identification of well-regulated promoters in many Proteobacteria and potentially beyond. Each plasmid is composed of four categories of genetic parts (i) the origin of replication, (ii) resistance marker, (iii) promoter-regulator and (iv) reporter. The plasmids can be efficiently assembled using ligation-independent cloning, and any gene of interest can be easily inserted in place of the reporter. We tested this toolbox in nine different Proteobacteria and identified regulated promoters with over fifty-fold induction range in eight of these bacteria. We also constructed variant libraries that enabled the identification of promoter-regulators with varied expression levels and increased inducible fold change relative to the original promoter. A selection of over 50 plasmids, which contain all of the toolbox's genetic parts, are available for community use and will enable easy construction and testing of genetics systems in both model and non-model bacteria.

A navigation guide of synthetic biology tools for Pseudomonas putida

Pseudomonas putida is a microbial chassis of huge potential for industrial and environmental biotechnology, owing to its remarkable metabolic versatility and ability to sustain difficult redox reactions and operational stresses, among other attractive characteristics. A wealth of genetic and in silico tools have been developed to enable the unravelling of its physiology and improvement of its performance. However, the rise of this microbe as a promising platform for biotechnological applications has resulted in diversification of tools and methods rather than standardization and convergence. As a consequence, multiple tools for the same purpose have been generated, whilst most of them have not been embraced by the scientific community, which has led to compartmentalization and inefficient use of resources. Inspired by this and by the substantial increase in popularity of P. putida, we aim herein to bring together and assess all currently available (wet and dry) synthetic biology tools specific for this microbe, focusing on the last 5 years. We provide information on the principles, functionality, advantages and limitations, with special focus on their use in metabolic engineering. Additionally, we compare the tool portfolio for P. putida with those for other bacterial chassis and discuss potential future directions for tool development. Therefore, this review is intended as a reference guide for experts and new 'users' of this promising chassis.

Engineering Pseudomonas putida KT2440 for the production of isobutanol

We engineered P. putida for the production of isobutanol from glucose by preventing product and precursor degradation, inactivation of the soluble transhydrogenase SthA, overexpression of the native ilvC and ilvD genes, and implementation of the feedback-resistant acetolactate synthase AlsS from Bacillus subtilis, ketoacid decarboxylase KivD from Lactococcus lactis, and aldehyde dehydrogenase YqhD from Escherichia coli. The resulting strain P. putida Iso2 produced isobutanol with a substrate specific product yield (Y Iso/S) of 22 ± 2 mg per gram of glucose under aerobic conditions. Furthermore, we identified the ketoacid decarboxylase from Carnobacterium maltaromaticum to be a suitable alternative for isobutanol production, since replacement of kivD from L. lactis in P. putida Iso2 by the variant from C. maltaromaticum yielded an identical YIso/S. Although P. putida is regarded as obligate aerobic, we show that under oxygen deprivation conditions this bacterium does not grow, remains metabolically active, and that engineered producer strains secreted isobutanol also under the non-growing conditions.

Efficient heterologous expression of nicotinate dehydrogenase in Comamonas testosteroni CNB-2 with transcriptional, folding enhancement strategy

Nicotinate dehydrogenase (NDHase) from Comamonas testosteroni JA1 catalyzes the C6 hydroxylation of 3-cyanopyridine with high regional selectivity, which is a very difficult and complex reaction for chemical synthesis. However, because NDHase is a membrane protein with three subunits (ndhS, ndhL and ndhM), it is difficult to express the enzyme in a functional form using common hosts such as Escherichia coli, Bacilus subtilis or Pichia pastoris. Furthermore, the enzyme requires special electron transfer chains in the membrane system for proper catalytic activity. Thus, we investigated the expression of NDHase in non-model bacterial strains, which are evolutionarily similar to C. testosteroni JA1, using several broad-host plasmids with different copy numbers as expression vectors. We successfully expressed NDHase in soluble from using the pVLT33 vector in C. testosteroni CNB-2, and found the activity of enzyme to be 40.6 U/L. To further improve the expression of NDHase in C. testosteroni CNB-2, we trialed a T7-like MmP1 system, composed of MmP1 RNA polymerase and an MmP1 promoter, which is used for transcriptional control in non-model bacteria. This increased protein expression and enzyme activity doubled to 90.5 U/L. A molecular chaperone was co-expressed using pBBR1 MCS-5 in the same host to improve the efficiency of folding and assembly of multi-subunit structures. The maximum activity was 115 U/L using the molecular chaperone GroES-EL, far surpassing the previously reported level, although expression was almost equivalent. These results indicate that a strategy involving the construction of a T7-like system and co-expression of a molecular chaperone offers an efficient approach for heterologous expression of enzymes that are difficult to express in functional forms using conventional hosts.

Genetic tools for reliable gene expression and recombineering in Pseudomonas putida

Pseudomonas putida is a promising bacterial host for producing natural products, such as polyketides and nonribosomal peptides. In these types of projects, researchers need a genetic toolbox consisting of plasmids, characterized promoters, and techniques for rapidly editing the genome. Past reports described constitutive promoter libraries, a suite of broad host range plasmids that replicate in P. putida, and genome-editing methods. To augment those tools, we have characterized a set of inducible promoters and discovered that IPTG-inducible promoter systems have poor dynamic range due to overexpression of the LacI repressor. By replacing the promoter driving lacI expression with weaker promoters, we increased the fold induction of an IPTG-inducible promoter in P. putida KT2440 to 80-fold. Upon discovering that gene expression from a plasmid was unpredictable when using a high-copy mutant of the BBR1 origin, we determined the copy numbers of several broad host range origins and found that plasmid copy numbers are significantly higher in P. putida KT2440 than in the synthetic biology workhorse, Escherichia coli. Lastly, we developed a λRed/Cas9 recombineering method in P. putida KT2440 using the genetic tools that we characterized. This method enabled the creation of scarless mutations without the need for performing classic two-step integration and marker removal protocols that depend on selection and counterselection genes. With the method, we generated four scarless deletions, three of which we were unable to create using a previously established genome-editing technique.

Pseudomonas putida KT2440 markerless gene deletion using a combination of λ Red recombineering and Cre/loxP site-specific recombination

Pseudomonas putida KT2440 is a saprophytic, environmental microorganism that plays important roles in the biodegradation of environmental toxic compounds and production of polymers, chemicals and secondary metabolites. Gene deletion of KT2440 usually involves cloning of the flanking homologous fragments of the gene of interest into a suicide vector followed by transferring into KT2440 via triparental conjugation. Selection and counterselection steps are then employed to generate gene deletion mutant. However, these methods are tedious and are not suitable for the manipulation of multiple genes simultaneously. Herein, a two-step, markerless gene deletion method is presented. First, homologous armsflanked loxP-neo-loxP was knocked-in to replace the gene of interest, then the kanamycin resistance marker is removed by Cre recombinase catalyzed site-specific recombination. Both two-plasmid and one-plasmid gene systems were established. MekR/PmekA regulated gene expression system was found to be suitable for tight Cre expression in one-plasmid deletion system. The straightforward, time saving and highly efficient markerless gene deletion strategy has the potential to facilitate the genetics and functional genomics study of P. putida KT2440.

Functional Characterization of the Mannitol Promoter of Pseudomonas fluorescens DSM 50106 and Its Application for a Mannitol-Inducible Expression System for Pseudomonas putida KT2440

A new pBBR1MCS-2-derived vector containing the Pseudomonas fluorescens DSM10506 mannitol promoter PmtlE and mtlR encoding its AraC/XylS type transcriptional activator was constructed and optimized for low basal expression. Mannitol, arabitol, and glucitol-inducible gene expression was demonstrated with Pseudomonas putida and eGFP as reporter gene. The new vector was applied for functional characterization of PmtlE. Identification of the DNA binding site of MtlR was achieved by in vivo eGFP measurement with PmtlE wild type and mutants thereof. Moreover, purified MtlR was applied for detailed in vitro investigations using electrophoretic mobility shift assays and DNaseI footprinting experiments. The obtained data suggest that MtlR binds to PmtlE as a dimer. The proposed DNA binding site of MtlR is AGTGC-N5-AGTAT-N7-AGTGC-N5-AGGAT. The transcription activation mechanism includes two binding sites with different binding affinities, a strong upstream binding site and a weaker downstream binding site. The presence of the weak downstream binding site was shown to be necessary to sustain mannitol-inducibility of PmtlE. Two possible functions of mannitol are discussed; the effector might stabilize binding of the second monomer to the downstream half site or promote transcription activation by inducing a conformational change of the regulator that influences the contact to the RNA polymerase.

Mining Environmental Plasmids for Synthetic Biology Parts and Devices

The scientific and technical ambition of contemporary synthetic biology is the engineering of biological objects with a degree of predictability comparable to those made through electric and industrial manufacturing. To this end, biological parts with given specifications are sequence-edited, standardized, and combined into devices, which are assembled into complete systems. This goal, however, faces the customary context dependency of biological ingredients and their amenability to mutation. Biological orthogonality (i.e., the ability to run a function in a fashion minimally influenced by the host) is thus a desirable trait in any deeply engineered construct. Promiscuous conjugative plasmids found in environmental bacteria have evolved precisely to autonomously deploy their encoded activities in a variety of hosts, and thus they become excellent sources of basic building blocks for genetic and metabolic circuits. In this article we review a number of such reusable functions that originated in environmental plasmids and keep their properties and functional parameters in a variety of hosts. The properties encoded in the corresponding sequences include inter alia origins of replication, DNA transfer machineries, toxin-antitoxin systems, antibiotic selection markers, site-specific recombinases, effector-dependent transcriptional regulators (with their cognate promoters), and metabolic genes and operons. Several of these sequences have been standardized as BioBricks and/or as components of the SEVA (Standard European Vector Architecture) collection. Such formatting facilitates their physical composability, which is aimed at designing and deploying complex genetic constructs with new-to-nature properties.