The presence of N-terminal secretion signal sequences leads to strong stimulation of the total expression levels of three tested medically important proteins during high-cell-density cultivations of Escherichia coli

Combinatorial mutagenesis and selection to understand and improve yeast promoters

Microbial promoters are important targets both for understanding the global gene expression and developing genetic tools for heterologous expression of proteins and complex biosynthetic pathways. Previously, we have developed and used combinatorial mutagenesis methods to analyse and improve bacterial expression systems. Here, we present for the first time an analogous strategy for yeast. Our model promoter is the strong and inducible P_AOX1 promoter in methylotrophic Pichia pastoris. The Zeocin resistance gene was applied as a valuable reporter for mutant P_AOX1 promoter activity, and we used an episomal plasmid vector to ensure a constant reporter gene dosage in the yeast host cells. This novel design enabled direct selection for colonies of recombinant cells with altered Zeocin tolerance levels originating solely from randomly introduced point mutations in the P_AOX1 promoter DNA sequence. We demonstrate that this approach can be used to select for P_AOX1 promoter variants with abolished glucose repression in large mutant libraries. We also selected P_AOX1 promoter variants with elevated expression level under induced conditions. The properties of the selected P_AOX1 promoter variants were confirmed by expressing luciferase as an alternative reporter gene. The tools developed here should be useful for effective screening, characterization, and improvement of any yeast promoters.

A comparative analysis of the properties of regulated promoter systems commonly used for recombinant gene expression in Escherichia coli

Background

Production of recombinant proteins in bacteria for academic and commercial purposes is a well established field; however the outcomes of process developments for specific proteins are still often unpredictable. One reason is the limited understanding of the performance of expression cassettes relative to each other due to different genetic contexts. Here we report the results of a systematic study aiming at exclusively comparing commonly used regulator/promoter systems by standardizing the designs of the replicon backbones.

Results

The vectors used in this study are based on either the RK2- or the pMB1- origin of replication and contain the regulator/promoter regions of XylS/Pm (wild-type), XylS/Pm ML1-17 (a Pm variant), LacI/P_T7lac, LacI/P_trc and AraC/P_BAD to control expression of different proteins with various origins. Generally and not unexpected high expression levels correlate with high replicon copy number and the LacI/P_T7lac system generates more transcript than all the four other cassettes. However, this transcriptional feature does not always lead to a correspondingly more efficient protein production, particularly if protein functionality is considered. In most cases the XylS/Pm ML1-17 and LacI/P_T7lac systems gave rise to the highest amounts of functional protein production, and the XylS/Pm ML1-17 is the most flexible in the sense that it does not require any specific features of the host. The AraC/P_BAD system is very good with respect to tightness, and a commonly used bioinformatics prediction tool (RBS calculator) suggested that it has the most translation-efficient UTR. Expression was also studied by flow cytometry in individual cells, and the results indicate that cell to cell heterogeneity is very relevant for understanding protein production at the population level.

Conclusions

The choice of expression system needs to be evaluated for each specific case, but we believe that the standardized vectors developed for this study can be used to more easily identify the nature of case-specific bottlenecks. By then taking into account the relevant characteristics of each expression cassette it will be easier to make the best choice with respect to the goal of achieving high levels of protein expression in functional or non-functional form.

Co-expression of two fibrolytic enzyme genes in CHO cells and transgenic mice

Cellulose is the main non-starch polysaccharides (NSP) in plant cell walls and acts as anti-nutritional factor in animal feed. However, monogastric animals do not synthesize enzymes that cleave such plant structural polysaccharides and thus waste of resources and pollute the environment. We described the vectors construction and co-expressions of a multi-functional cellulase EGX (with the activities of exo-β-1,4-glucanase, endo-β-1,4-glucanase, and endo-β-1,4-xylanase activities) from mollusca, Ampullaria crossean and a β-glucosidase BGL1 from Asperjillus niger in CHO cells and the transgenic mice. The recombinant enzymes were synthesised, secreted by the direction of pig PSP signal peptide and functionally active in the eukaryote systems including both of CHO cells and transgenic mice by RT-PCR analysis, western blot analysis and cellulolytic enzymes activities assays. Expressions were salivary glands-specific dependent under the control of pig PSP promoter in transgenic mice. 2A peptide was used as the self-cleaving sequence to mediate co-expression of the fusion genes and the cleavage efficiency was very high both in vitro and in vivo according to the western blot analysis. In summary, we have demonstrated that the single ORF containing EGX and BGL1 were co-expressed by 2A peptide in CHO cells and transgenic mice. It presents a viable technology for efficient disruption of plant cell wall and liberation of nutrients. To our knowledge, this is the first report using 2A sequence to produce multiple cellulases in mammalian cells and transgenic animals.

Combinatorial mutagenesis and selection of improved signal sequences and their application for high-level production of translocated heterologous proteins in Escherichia coli

We previously designed the consensus signal peptide (CSP) and demonstrated that it can be used to strongly stimulate heterologous protein production in Escherichia coli. A comparative study using CSP and two bacterial signal sequences, pelB and ompA, showed that the effect of signal sequences on both expression level and translocation efficiency can be highly protein specific. We report here the generation of CSP mutant libraries by a combinatorial mutagenesis approach. Degenerated CSP oligonucleotides were cloned in frame with the 5' end of the bla gene, encoding the mature periplasmic β-lactamase released from its native signal sequence. This novel design allows for a direct selection of improved signal sequences that positively affect the expression level and/or translocation efficiency of β-lactamase, based on the ampicillin tolerance level of the E. coli host cells. By using this strategy, 61 different CSP mutants with up to 8-fold-increased ampicillin tolerance level and up to 5.5-fold-increased β-lactamase expression level were isolated and characterized genetically. A subset of the CSP mutants was then tested with the alternative reporter gene phoA, encoding periplasmic alkaline phosphatase (AP), resulting in an up to 8-fold-increased production level of active AP protein in E. coli. Moreover, it was demonstrated that the CSP mutants can improve the production of the medically important human interferon α2b under high-cell-density cultivations. Our results show that there is a clear potential for improving bacterial signal sequences by using combinatorial mutagenesis, and bioinformatics analyses indicated that the beneficial mutations could not be rationally predicted.

Strong stimulation of recombinant protein production in Escherichia coli by combining stimulatory control elements in an expression cassette

Background

The XylS/Pm expression system has been used to produce recombinant proteins at industrial levels in Escherichia coli. Activation of transcription from the Pm promoter takes place in the presence of benzoic acid or derivatives of it. Previous mutagenesis studies resulted in identification of several variants of the expression control elements xylS (X), Pm (P) and the 5'-untranslated region (U) that individually gave rise to strongly stimulated expression. The goal of this study was to test if combination of such stimulatory mutations in the same expression vectors would lead to further increase of expression levels.

Results

We combined X, P and U variants that were originally identified due to their ability to strongly stimulate expression of the reporter gene bla (resistance to penicillin). Combination of optimized elements stimulated bla expression up to 75-fold (X, P and U combined) relative to the wild-type system, while accumulated transcript levels increased about 50-fold. This is much more than for the elements individually. We also tested combination of the variant elements on two other and unrelated genes, celB (encoding phosphoglucomutase) and the human growth factor gene gm-csf. Protein production from these genes is much more efficient than from bla in the wild-type system, but expression was still significantly stimulated by the combination of X, P and U variants, although not to the same extent as for bla.

We also integrated a single copy of the expression cassette with each gene into the E. coli chromosome and found that the expression level from this single copy was higher for bla than for the wild-type plasmid system, while it was lower for celB and gm-csf.

Conclusion

Our results show that combination of stimulatory expression control elements can be used to further increase production of different proteins in E. coli. For one reporter gene (bla) this allowed for more protein production from a single gene copy integrated on the chromosome, compared to the wild-type plasmid system. The approach described here should in principle be applicable for improvement of any expression cassette.

Exploring the 5'-UTR DNA region as a target for optimizing recombinant gene expression from the strong and inducible Pm promoter in Escherichia coli

By using the strong and inducible Pm promoter as a model, we recently reported that the β-lactamase production (encoded by bla) can be stimulated up to 20-fold in Escherichia coli by mutating the DNA region corresponding to the 5'-untranslated region of mRNA (UTR). One striking observation was the unexpected large stimulatory effect some of these UTR variants had on the bla transcript production level. We here demonstrate that such UTR variants can also be used to improve the expression level of the alternative genes celB (encoding phosphoglucomutase) and inf-α2b (encoding human cytokine interferon α2b), which both can be expressed to high levels even with the wild-type Pm UTR DNA sequence. Our data indicated some degree of context dependency between the UTR DNA and concomitant recombinant gene sequences. By constructing and using a synthetic operon, we demonstrated that UTR variants optimized for high-level expression of probably any recombinant gene can be efficiently selected from large UTR mutant libraries. The stimulation affected both the transcript production and translational level, and such modified UTR sequences therefore clearly have a significant applied potential for improvement of recombinant gene expression processes.

The expression of recombinant genes in Escherichia coli can be strongly stimulated at the transcript production level by mutating the DNA-region corresponding to the 5'-untranslated part of mRNA

Secondary structures and the short Shine-Dalgarno sequence in the 5'-untranslated region of bacterial mRNAs (UTR) are known to affect gene expression at the level of translation. Here we report the use of random combinatorial DNA sequence libraries to study UTR function, applying the strong, σ(32)/σ(38)-dependent, and positively regulated Pm promoter as a model. All mutations in the libraries are located at least 8 bp downstream of the transcriptional start site. The libraries were screened using the ampicillin-resistance gene (bla) as reporter, allowing easy identification of UTR mutants that display high levels of expression (up to 20-fold increase relative to the wild-type at the protein level). Studies of the two UTR mutants identified by a modified screening procedure showed that their expression is stimulated to a similar extent at both the transcript and protein product levels. For one such mutant a model analysis of the transcription kinetics showed significant evidence of a difference in the transcription rate (about 18-fold higher than the wild type), while there was no evidence of a difference in transcript stability. The two UTR sequences also stimulated expression from a constitutive σ(70)-dependent promoter (P1/P(anti-tet)), demonstrating that the UTR at the DNA or RNA level has a hitherto unrecognized role in transcription.

Cloning, soluble expression and purification of high yield recombinant hGMCSF in Escherichia coli

Expression of human granulocyte macrophage colony stimulating factor (hGMCSF), a cytokine of therapeutic importance, as a thioredoxin (TRX) fusion has been investigated in Escherichia coli BL21 (DE3) codon plus cells. The expression of this protein was low when cloned under the T7 promoter without any fusion tags. High yield of GMCSF was achieved (∼88 mg/L of fermentation broth) in the shake flask when the gene was fused to the E. coli TRX gene. The protein was purified using a single step Ni(2+)-NTA affinity chromatography and the column bound fusion tag was removed by on-column cleavage with enterokinase. The recombinant hGMCSF was expressed as a soluble and biologically active protein in E. coli, and upon purification, the final yield was ∼44 mg/L in shake flask with a specific activity of 2.3 × 10(8) U/mg. The results of Western blot and RP-HPLC analyses, along with biological activity using the TF-1 cell line, established the identity of the purified hGMCSF. In this paper, we report the highest yield of hGMCSF expressed in E. coli. The bioreactor study shows that the yield of hGMCSF could be easily scalable with a yield of ∼400 mg/L, opening up new opportunities for large scale production hGMCSF in E. coli.

Continuous control of the flow in biochemical pathways through 5' untranslated region sequence modifications in mRNA expressed from the broad-host-range promoter Pm

The inducible Pm promoter integrated into broad-host-range plasmid RK2 replicons can be fine-tuned continuously between the uninduced and maximally induced levels by varying the inducer concentrations. To lower the uninduced background level while still maintaining the inducibility for applications in, for example, metabolic engineering and synthetic (systems) biology, we report here the use of mutations in the Pm DNA region corresponding to the 5' untranslated region of mRNA (UTR). Five UTR variants obtained by doped oligonucleotide mutagenesis and selection, apparently reducing the efficiency of translation, were all found to display strongly reduced uninduced expression of three different reporter genes (encoding β-lactamase, luciferase, and phosphoglucomutase) in Escherichia coli. The ratio between induced and uninduced expression remained the same or higher compared to cells containing a corresponding plasmid with the wild-type UTR. Interestingly, the UTR variants also displayed similar effects on expression when substituted for the native UTR in another and constitutive promoter, P1 (P(antitet)), indicating a broad application potential of these UTR variants. Two of the selected variants were used to control the production of the C(50) carotenoid sarcinaxanthin in an engineered strain of E. coli that produces the precursor lycopene. Sarcinaxanthin is produced in this particular strain by expressing three Micrococcus luteus derived genes from the promoter Pm. The results indicated that UTR variants can be used to eliminate sarcinaxanthin production under uninduced conditions, whereas cells containing the corresponding plasmid with a wild-type UTR produced ca. 25% of the level observed under induced conditions.

Isolation of mutant alginate lyases with cleavage specificity for di-guluronic acid linkages

Alginates are commercially valuable and complex polysaccharides composed of varying amounts and distribution patterns of 1-4-linked β-D-mannuronic acid (M) and α-L-guluronic acid (G). This structural variability strongly affects polymer physicochemical properties and thereby both commercial applications and biological functions. One promising approach to alginate fine structure elucidation involves the use of alginate lyases, which degrade the polysaccharide by cleaving the glycosidic linkages through a β-elimination reaction. For such studies one would ideally like to have different lyases, each of which cleaves only one of the four possible linkages in alginates: G-G, G-M, M-G, and M-M. So far no lyase specific for only G-G linkages has been described, and here we report the construction of such an enzyme by mutating the gene encoding Klebsiella pneumoniae lyase AlyA (a polysaccharide lyase family 7 lyase), which cleaves both G-G and G-M linkages. After error-prone PCR mutagenesis and high throughput screening of ∼7000 lyase mutants, enzyme variants with a strongly improved G-G specificity were identified. Furthermore, in the absence of Ca(2+), one of these lyases (AlyA5) was found to display no detectable activity against G-M linkages. G-G linkages were cleaved with ∼10% of the optimal activity under the same conditions. The substitutions conferring altered specificity to the mutant enzymes are located in conserved regions in the polysaccharide lyase family 7 alginate lyases. Structure-function analyses by comparison with the known three-dimensional structure of Sphingomonas sp. A1 lyase A1-II' suggests that the improved G-G specificity might be caused by increased affinity for nonproductive binding of the alternating G-M structure.

Comparison of expression systems for the production of human interferon-α2b

The production of human interferon alpha2b (IFN-α2b) in two expression systems, tobacco (Nicotiana tabaccum) and Escherichia coli, was compared in various aspects such as safety, yield, quality of product and productivity. In the E. coli system, IFN-α2b was expressed under a pelB signal sequence and a T7lac promoter in a pET 26b(+) vector. The same gene was also cloned in expression plant vector (pCAMBIA1304) between cauliflower mosaic virus promoter (CaMV35S) and poly A termination region (Nos) and expressed in transgenic tobacco plants. The expression of protein in both systems was confirmed by western immunoblotting and the quantity of the protein was determined by immunoassay. The amount of periplasmic expression in E. coli was 60 µg/L of culture, while the amount of nuclear expression in the plant was 4.46 µg/kg of fresh leaves. The result of this study demonstrated that IFN-α2b was successfully expressed in periplasm of bacterial and plant systems. The limitations on the production of IFN-α2b by both systems are addressed and discussed to form the basis for the selection of the appropriate expression platform.

The human alpha(2)-plasmin inhibitor: functional characterization of the unique plasmin(ogen)-binding region

The human alpha(2)-plasmin inhibitor (A2PI) possesses unique N- and C-terminal extensions that significantly influence its biological activities. The C-terminal segment, A2PIC (Asn(398)-Lys(452)), contains six lysines thought to be involved in the binding to lysine-binding sites in the kringle domains of human plasminogen, of which four (Lys(422), Lys(429), Lys(436), Lys(452)) are completely and two (Lys(406), Lys(415)) are partially conserved. Multiple Lys to Ala mutants of A2PIC were expressed in Escherichia coli and used in intrinsic fluorescence titrations with kringle domains K1, K4, K4 + 5, and K1 + 2 + 3 of human plasminogen. We were able to identify the C-terminal Lys(452) as the main binding partner in recombinant A2PIC (rA2PIC) constructs with isolated kringles. We could show a cooperative, zipper-like enhancement of the interaction between C-terminal Lys(452) and internal Lys(436) of rA2PIC and isolated K1 + 2 + 3, whereas the other internal lysine residues contribute only to a minor extent to the binding process. Sulfated Tyr(445) in the unique C-terminal segment revealed no influence on the binding affinity to kringle domains.

Directed evolution of the transcription factor XylS for development of improved expression systems

The inducible Pm promoter together with its cognate positive transcription regulator XylS has been shown to be useful for recombinant protein production under high cell density conditions. Here we report directed evolution of XylS resulting in mutant proteins with increased ability to stimulate transcription in Escherichia coli from Pm. A first round of mutagenesis using error-prone PCR on xylS was used to construct a library consisting of about 430,000 clones, and this library could be efficiently screened with respect to stimulation of expression from Pm due to a positive correlation between the level of expression of the reporter gene, bla (encoding β-lactamase), and the ampicillin tolerance of the corresponding host cells. Fourteen different amino acid substitutions in XylS were found to separately lead to up to nearly a threefold stimulation of expression under induced conditions, relative to wild type. These mutations were all located in the part corresponding to the N-terminal half of the protein. Varying combinations of the mutations resulted in further stimulation, and the best results (about 10-fold stimulation under induced conditions) were obtained by using a random shuffling procedure followed by a new round of screening. The uninduced levels of expression for the same mutants also increased, but only about four times. Through in silico 3D modelling of the N-terminal domain of XylS, it was observed that the evolved mutant proteins contained substitutions that were positioned in different parts of the predicted structure, including a β-barrel putatively responsible for effector binding and a coiled coil probably important for dimerization. The total production of the host-toxic antibody fragment scFv-phOx expressed from Pm with the evolved XylS mutant protein StEP-13 was about ninefold higher than with wild-type XylS, demonstrating that directed evolution of transcription factors can be an important new tool to achieve high-level recombinant protein production.

Strategies for successful recombinant expression of disulfide bond-dependent proteins in Escherichia coli

Bacteria are simple and cost effective hosts for producing recombinant proteins. However, their physiological features may limit their use for obtaining in native form proteins of some specific structural classes, such as for instance polypeptides that undergo extensive post-translational modifications. To some extent, also the production of proteins that depending on disulfide bridges for their stability has been considered difficult in E. coli.

Both eukaryotic and prokaryotic organisms keep their cytoplasm reduced and, consequently, disulfide bond formation is impaired in this subcellular compartment. Disulfide bridges can stabilize protein structure and are often present in high abundance in secreted proteins. In eukaryotic cells such bonds are formed in the oxidizing environment of endoplasmic reticulum during the export process. Bacteria do not possess a similar specialized subcellular compartment, but they have both export systems and enzymatic activities aimed at the formation and at the quality control of disulfide bonds in the oxidizing periplasm.

This article reviews the available strategies for exploiting the physiological mechanisms of bactera to produce properly folded disulfide-bonded proteins.

Random mutagenesis of the PM promoter as a powerful strategy for improvement of recombinant-gene expression

The inducible Pm-xylS promoter system has proven useful for production of recombinant proteins in several gram-negative species and in high-cell-density cultivations of Escherichia coli. In this study we subjected a 24-bp region of Pm (including the -10 element) to random mutagenesis, leading to large mutant libraries in E. coli. Low-frequency-occurring Pm mutants displaying strongly increased promoter activity (up-mutants) could be efficiently identified by using beta-lactamase as a reporter. The up-mutants typically carried multiple point mutations positioned throughout the mutagenized region, combined with deletions around the transcription start site. Mutants displaying up to about a 14-fold increase in beta-lactamase expression (relative to wild-type Pm) were identified without loss of the inducible phenotype. The mutants also strongly stimulated the expression of two other reporter genes, luc (encoding firefly luciferase) and celB (encoding phosphoglucomutase), and were found to significantly improve (twofold) a previously optimized process for high-level recombinant production of the medically important granulocyte-macrophage colony-stimulating factor in E. coli under high-cell-density conditions. These results demonstrate the potential of using random mutagenesis of promoters to improve protein expression at industrial levels and indicate that targeted modifications of individual functional elements are not sufficient to obtain optimized promoter sequences.

Positively regulated bacterial expression systems

Regulated promoters are useful tools for many aspects related to recombinant gene expression in bacteria, including for high‐level expression of heterologous proteins and for expression at physiological levels in metabolic engineering applications. In general, it is common to express the genes of interest from an inducible promoter controlled either by a positive regulator or by a repressor protein. In this review, we discuss established and potentially useful positively regulated bacterial promoter systems, with a particular emphasis on those that are controlled by the AraC‐XylS family of transcriptional activators. The systems function in a wide range of microorganisms, including enterobacteria, soil bacteria, lactic bacteria and streptomycetes. The available systems that have been applied to express heterologous genes are regulated either by sugars (l‐arabinose, l‐rhamnose, xylose and sucrose), substituted benzenes, cyclohexanone‐related compounds, ε‐caprolactam, propionate, thiostrepton, alkanes or peptides. It is of applied interest that some of the inducers require the presence of transport systems, some are more prone than others to become metabolized by the host and some have been applied mainly in one or a limited number of species. Based on bioinformatics analyses, the AraC‐XylS family of regulators contains a large number of different members (currently over 300), but only a small fraction of these, the XylS/Pm, AraC/P_BAD, RhaR‐RhaS/rhaBAD, NitR/PnitA and ChnR/Pb regulator/promoter systems, have so far been explored for biotechnological applications.

Bacterial-based systems for expression and purification of recombinant Lassa virus proteins of immunological relevance

BACKGROUND

There is a significant requirement for the development and acquisition of reagents that will facilitate effective diagnosis, treatment, and prevention of Lassa fever. In this regard, recombinant Lassa virus (LASV) proteins may serve as valuable tools in diverse antiviral applications. Bacterial-based systems were engineered for expression and purification of recombinant LASV nucleoprotein (NP), glycoprotein 1 (GP1), and glycoprotein 2 (GP2).

RESULTS

Full-length NP and the ectodomains of GP1 and GP2 were generated as maltose-binding protein (MBP) fusions in the Rosetta strains of Escherichia coli (E. coli) using pMAL-c2x vectors. Average fusion protein yields per liter of culture for MBP-NP, MBP-GP1, and MBP-GP2 were 10 mg, 9 mg, and 9 mg, respectively. Each protein was captured from cell lysates using amylose resin, cleaved with Factor Xa, and purified using size-exclusion chromatography (SEC). Fermentation cultures resulted in average yields per liter of 1.6 mg, 1.5 mg, and 0.7 mg of purified NP, GP1 and GP2, respectively. LASV-specific antibodies in human convalescent sera specifically detected each of the purified recombinant LASV proteins, highlighting their utility in diagnostic applications. In addition, mouse hyperimmune ascitic fluids (MHAF) against a panel of Old and New World arenaviruses demonstrated selective cross reactivity with LASV proteins in Western blot and enzyme-linked immunosorbent assay (ELISA).

CONCLUSION

These results demonstrate the potential for developing broadly reactive immunological assays that employ all three arenaviral proteins individually and in combination.

Analysis of type II secretion of recombinant pneumococcal PspA and PspC in a Salmonella enterica serovar Typhimurium vaccine with regulated delayed antigen synthesis

Recombinant attenuated Salmonella vaccines (RASVs) have been used extensively to express and deliver heterologous antigens to host mucosal tissues. Immune responses can be enhanced greatly when the antigen is secreted to the periplasm or extracellular compartment. The most common method for accomplishing this is by fusion of the antigen to a secretion signal sequence. Finding an optimal signal sequence is typically done empirically. To facilitate this process, we constructed a series of plasmid expression vectors, each containing a different type II signal sequence. We evaluated the utilities of these vectors by fusing two different antigens, the alpha-helix domains of pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC), to the signal sequences of beta-lactamase (bla SS), ompA, and phoA and the signal sequence and C-terminal peptide of beta-lactamase (bla SS+CT) on Asd(+) plasmids under the control of the P(trc) promoter. Strains were characterized for level of expression, subcellular antigen location, and the capacity to elicit antigen-specific immune responses and protection against challenge with Streptococcus pneumoniae in mice. The immune responses to each protein differed depending on the signal sequence used. Strains carrying the bla SS-pspA and bla SS+CT-pspC fusions yielded the largest amounts of secreted PspA and PspC, respectively, and induced the highest serum IgG titers, although all fusion proteins tested induced some level of antigen-specific IgG response. Consistent with the serum antibody responses, RASVs expressing the bla SS-pspA and bla SS+CT-pspC fusions induced the greatest protection against S. pneumoniae challenge.

The Acinetobacter sp. chnB promoter together with its cognate positive regulator ChnR is an attractive new candidate for metabolic engineering applications in bacteria

Over the last 10 years there has been an extremely fast development in the global characterization of bacteria at the genome, transcriptome, proteome and metabolome levels. To further explore and apply these complex data sets there is now a need for new biological tools that can be used to test or verify hypotheses generated on the basis of all the new information. Here, we report the integration of an expression cassette based on the Acinetobacter sp. chnB promoter and its cognate positive regulator chnR gene into a replicon derived from the broad-host-range plasmid RK2. Cyclohexanone was found to be the most efficient inducer of this system in Escherichia coli, using firefly luciferase as a reporter. To explore the potential of the system in another species, we show that the system can be used in combination with another similar expression cassette (Pm/xylS) to control the monomer composition of the industrially widely used exopolysaccharide alginate, produced by Pseudomonas fluorescens.