Rational design of a bacterial transcriptional cascade for amplifying gene expression capacity

A bacterial sialidase mediates early-life colonization by a pioneering gut commensal

The early microbial colonization of the gastrointestinal tract can have long-term impacts on development and health. Keystone species, including Bacteroides spp., are prominent in early life and play crucial roles in maintaining the structure of the intestinal ecosystem. However, the process by which a resilient community is curated during early life remains inadequately understood. Here, we show that a single sialidase, NanH, in Bacteroides fragilis mediates stable occupancy of the intestinal mucosa in early life and regulates a commensal colonization program. This program is triggered by sialylated glycans, including those found in human milk oligosaccharides and intestinal mucus. NanH is required for vertical transmission from dams to pups and promotes B. fragilis dominance during early life. Furthermore, NanH facilitates commensal resilience and recovery after antibiotic treatment in a defined microbial community. Collectively, our study reveals a co-evolutionary mechanism between the host and microbiota mediated through host-derived glycans to promote stable colonization.

Construction of cascade circuits for dynamic temporal regulation and its application to PHB production

BACKGROUND

To maximize the production capacity and yield of microbial cell factories, metabolic pathways are generally modified with dynamic regulatory strategies, which can effectively solve the problems of low biological yield, growth retardation and metabolic imbalance. However, the strategy of dynamic regulating multiple genes in different time and order is still not effectively solved. Based on the quorum-sensing (QS) system and the principle of cascade regulation, we studied the sequence and time interval of gene expression in metabolic pathways.

RESULTS

We designed and constructed a self-induced dynamic temporal regulatory cascade circuit in Escherichia coli using the QS system and dual regulatory protein cascade and found that the time intervals of the cascade circuits based on the Tra, Las system and the Lux, Tra system reached 200 min and 150 min, respectively. Furthermore, a dynamic temporal regulatory cascade circuit library with time intervals ranging from 110 to 310 min was obtained based on this circuit using promoter engineering and ribosome binding site replacement, which can provide more selective synthetic biology universal components for metabolic applications. Finally, poly-β-hydroxybutyric acid (PHB) production was taken as an example to demonstrate the performance of the cascade circuit library. The content of PHB increased 1.5-fold. Moreover, circuits with different time intervals and different expression orders were found to have different potentials for application in PHB production, and the preferred time-interval circuit strain C2-max was identified by screening.

CONCLUSIONS

The self-induced dynamic temporal regulation cascade circuit library can enable the expression of target genes with sequential changes at different times, effectively solving the balance problem between cell growth and product synthesis in two-stage fermentation and expanding the application of dynamic regulatory strategies in the field of metabolic engineering.

Recent advances in bacterial therapeutics based on sense and response

Intelligent drug delivery is a promising strategy for cancer therapies. In recent years, with the rapid development of synthetic biology, some properties of bacteria, such as gene operability, excellent tumor colonization ability, and host-independent structure, make them ideal intelligent drug carriers and have attracted extensive attention. By implanting condition-responsive elements or gene circuits into bacteria, they can synthesize or release drugs by sensing stimuli. Therefore, compared with traditional drug delivery, the usage of bacteria for drug loading has better targeting ability and controllability, and can cope with the complex delivery environment of the body to achieve the intelligent delivery of drugs. This review mainly introduces the development of bacterial-based drug delivery carriers, including mechanisms of bacterial targeting to tumor colonization, gene deletions or mutations, environment-responsive elements, and gene circuits. Meanwhile, we summarize the challenges and prospects faced by bacteria in clinical research, and hope to provide ideas for clinical translation.

The XylS/Pm regulator/promoter system and its use in fundamental studies of bacterial gene expression, recombinant protein production and metabolic engineering

The XylS/Pm regulator/promoter system originating from the Pseudomonas putida TOL plasmid pWW0 is widely used for regulated low‐ and high‐level recombinant expression of genes and gene clusters in Escherichia coli and other bacteria. Induction of this system can be graded by using different cheap benzoic acid derivatives, which enter cells by passive diffusion, operate in a dose‐dependent manner and are typically not metabolized by the host cells. Combinatorial mutagenesis and selection using the bla gene encoding β‐lactamase as a reporter have demonstrated that the Pm promoter, the DNA sequence corresponding to the 5′ untranslated end of its cognate mRNA and the xylS coding region can be modified and improved relative to various types of applications. By combining such mutant genetic elements, altered and extended expression profiles were achieved. Due to their unique properties, obtained systems serve as a genetic toolbox valuable for heterologous protein production and metabolic engineering, as well as for basic studies aiming at understanding fundamental parameters affecting bacterial gene expression. The approaches used to modify XylS/Pm should be adaptable for similar improvements also of other microbial expression systems. In this review, we summarize constructions, characteristics, refinements and applications of expression tools using the XylS/Pm system.

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.

Widening functional boundaries of the σ(54) promoter Pu of Pseudomonas putida by defeating extant physiological constraints

The extant layout of the σ(54) promoter Pu, harboured by the catabolic TOL plasmid, pWW0, of Pseudomonas putida is one of the most complex instances of endogenous and exogenous signal integration known in the prokaryotic domain. In this regulatory system, all signal inputs are eventually translated into occupation of the promoter sequence by either of two necessary components: the m-xylene responsive transcriptional factor XylR and the σ(54) containing form of RNA polymerase. Modelling of these components indicated that the Pu promoter could be upgraded to respond with much greater capacity to aromatic inducers by artificially increasing the endogenous levels of both XylR and the σ(54) sigma factor, either separately or together. To explore these scenarios, expression of rpoN, the gene encoding σ(54), was placed under the control of an orthogonal regulatory system that was inducible by salicylic acid. We generated a knock-in P. putida strain containing this construct alongside the xylR/Pu regulatory module in its native configuration, and furthermore, a second strain where xylR expression was under the control of an engineered positive-feedback loop. These interventions allowed us to dramatically increase the transcriptional capacity (i.e. absolute promoter output) of Pu far beyond its natural scope. In addition, they resulted in a new regulatory device displaying more sensitive and ultra-fast responses to m-xylene. To our knowledge, this is the first time that the working regime of a promoter has been rationally modified by releasing the constraints imposed by its innate constituents.

Regulation of the expression level of transcription factor XylS reveals new functional insight into its induction mechanism at the Pm promoter

BACKGROUND

XylS is the positive regulator of the inducible Pm promoter, originating from Pseudomonas putida, where the system controls a biochemical pathway involved in degradation of aromatic hydrocarbons, which also act as inducers. The XylS/Pm positive regulator/promoter system is used for recombinant gene expression and the output from Pm is known to be sensitive to the intracellular XylS concentration.

RESULTS

By constructing a synthetic operon consisting of xylS and luc, the gene encoding luciferase, relative XylS expression levels could be monitored indirectly at physiological concentrations. Expression of XylS from inducible promoters allowed control over a more than 800-fold range, however, the corresponding output from Pm covered only an about five-fold range. The maximum output from Pm could not be increased by introducing more copies of the promoter in the cells. Interestingly, a previously reported XylS variant (StEP-13), known to strongly stimulate expression from Pm, caused the same maximum activity from Pm as wild-type XylS at high XylS expression levels. Under uninduced conditions expression from Pm also increased as a function of XylS expression levels, and at very high concentrations the maximum activity from Pm was the same as in the presence of inducer.

CONCLUSION

According to our proposed model, which is in agreement with current knowledge, the regulator, XylS, can exist in three states: monomers, dimers, and aggregates. Only the dimers are active and able to induce expression from Pm. Their maximum intracellular concentration and the corresponding output from Pm are limited by the concentration-dependent conversion into inactive aggregates. Maximization of the induction ratio at Pm can be obtained by expression of XylS at the level where aggregation occurs, which might be exploited for recombinant gene expression. The results described here also indicate that there might exist variants of XylS which can exist at higher active dimer concentrations and thus lead to increased expression levels from Pm.

Novel tools to analyze the function of Salmonella effectors show that SvpB ectopic expression induces cell cycle arrest in tumor cells

In order to further characterize its role in pathogenesis and to establish whether its overproduction can lead to eukaryotic tumor cell death, Salmonella strains able to express its virulence factor SpvB (an ADP-ribosyl transferase enzyme) in a salicylate-inducible way have been constructed and analyzed in different eukaryotic tumor cell lines. To do so, the bacterial strains bearing the expression system have been constructed in a ∆purD background, which allows control of bacterial proliferation inside the eukaryotic cell. In the absence of bacterial proliferation, salicylate-induced SpvB production resulted in activation of caspases 3 and 7 and apoptotic cell death. The results clearly indicated that controlled SpvB production leads to F-actin depolimerization and either G1/S or G2/M phase arrest in all cell lines tested, thus shedding light on the function of SpvB in Salmonella pathogenesis. In the first place, the combined control of protein production by salicylate regulated vectors and bacterial growth by adenine concentration offers the possibility to study the role of Salmonella effectors during eukaryotic cells infection. In the second place, the salicylate-controlled expression of SpvB by the bacterium provides a way to evaluate the potential of other homologous or heterologous proteins as antitumor agents, and, eventually to construct novel potential tools for cancer therapy, given that Salmonella preferentially proliferates in tumors.

Zebrafish as a model organism to study host-pathogen interactions

Zebrafish have been extensively used in biomedical research as a model to study vertebrate development but it is only recently that it has also been adopted into varied fields such as immunology and host-pathogen interactions. Zebrafish have a rapid life cycle, small size and the adults exhibit no territorial behavior in relatively dense cages. Under standard conditions each female lays an average of a hundred eggs per clutch, providing a large number of larvae per week. Their transparency during early life stages allows real time visualization of the different organs, which makes them especially suitable for the study of bacterial host-pathogen interactions. Traditionally, these studies have been technically challenging in higher organisms, given the loss of control over the bacteria once the pathogen infects its host. Here we describe an emerging approach to monitor Salmonella typhimurium infection progression using in vivo fluorescence upon parenteral infection. We have engineered Salmonella with the Cascade expression system; an efficient method to voluntarily activate bacterial heterologous gene expression at any point during infection once inside the Zebrafish macrophages, using a non-toxic inducer.

Heterologous viral expression systems in fosmid vectors increase the functional analysis potential of metagenomic libraries

The extraordinary potential of metagenomic functional analyses to identify activities of interest present in uncultured microorganisms has been limited by reduced gene expression in surrogate hosts. We have developed vectors and specialized E. coli strains as improved metagenomic DNA heterologous expression systems, taking advantage of viral components that prevent transcription termination at metagenomic terminators. One of the systems uses the phage T7 RNA-polymerase to drive metagenomic gene expression, while the other approach uses the lambda phage transcription anti-termination protein N to limit transcription termination. A metagenomic library was constructed and functionally screened to identify genes conferring carbenicillin resistance to E. coli. The use of these enhanced expression systems resulted in a 6-fold increase in the frequency of carbenicillin resistant clones. Subcloning and sequence analysis showed that, besides β-lactamases, efflux pumps are not only able contribute to carbenicillin resistance but may in fact be sufficient by themselves to convey carbenicillin resistance.

Improved expression systems for regulated expression in Salmonella infecting eukaryotic cells

In this work we describe a series of improvements to the Salmonella-based salicylate-inducible cascade expression system comprised of a plasmid-borne expression module, where target gene expression is driven by the P(m) promoter governed by the XylS2 regulator, and a genome-integrated regulatory module controlled by the nahR/P(sal) system. We have constructed a set of high and low-copy number plasmids bearing modified versions of the expression module with a more versatile multiple cloning site and different combinations of the following elements: (i) the nasF transcriptional attenuator, which reduces basal expression levels, (ii) a strong ribosome binding site, and (iii) the Type III Secretion System (TTSS) signal peptide from the effector protein SspH2 to deliver proteins directly to the eukaryotic cytosol following bacterial infection of animal cells. We show that different expression module versions can be used to direct a broad range of protein production levels. Furthermore, we demonstrate that the efficient reduction of basal expression by the nasF attenuator allows the cloning of genes encoding highly cytotoxic proteins such as colicin E3 even in the absence of its immunity protein. Additionally, we show that the Salmonella TTSS is able to translocate most of the protein produced by this regulatory cascade to the cytoplasm of infected HeLa cells. Our results indicate that these vectors represent useful tools for the regulated overproduction of heterologous proteins in bacterial culture or in animal cells, for the cloning and expression of genes encoding toxic proteins and for pathogenesis studies.

Exploitation of prokaryotic expression systems based on the salicylate-dependent control circuit encompassing nahR/P(sal)::xylS2 for biotechnological applications

Expression vectors appear to be an indispensable tool for both biological studies and biotechnological applications. Controlling gene overexpression becomes a critical issue when protein production is desired. In addition to several aspects regarding toxicity or plasmid instability, tight control of gene expression is an essential factor in biotechnological processes. Thus, the search for better-controlled circuits is an important issue among biotechnologists. Traditionally, expression systems involve a single regulatory protein operating over a target promoter. However, these circuits are limited on their induction ratios (e.g., by their restriction in the maximal expression capacity, by their leakiness under non-induced conditions). Due to these limitations, regulatory cascades, which are far more efficient, are necessary for biotechnological applications. Thus, regulatory circuits with two modules operating in cascade offer a significant advantage. In this review, we describe the regulatory cascade based on two salicylate-responsive transcriptional regulators of Pseudomonas putida (nahR/P(sal)::xylS2), its properties, and contribution to a tighter control over heterologous gene expression in different applications.Nowadays, heterologous expression has been proven to be an indispensable tool for tackling basic biological questions, as well as for developing biotechnological applications. As the nature of the protein of interest becomes more complex, biotechnologists find that a tight control of gene expression is a key factor which conditions the success of the downstream purification process, as well as the interpretation of the results in other type of studies. Fortunately, different expression systems can be found in the market, each of them with their own pros and cons. In this review we discuss the exploitation of prokaryotic expression systems based on a promising expression system, the salicylate-dependent control circuit encompassing nahR/P(sal)::xylS2, as well as some of the improvements that have been done on this system to exploit it more efficiently in the context of both biotechnological applications and basic research.

3-Hydroxyphenylpropionate and phenylpropionate are synergistic activators of the MhpR transcriptional regulator from Escherichia coli

The degradation of the aromatic compound phenylpropionate (PP) in Escherichia coli K-12 requires the activation of two different catabolic pathways coded by the hca and the mhp gene clusters involved in the mineralization of PP and 3-hydroxyphenylpropionate (3HPP), respectively. The compound 3-(2,3-dihydroxyphenyl)propionate (DHPP) is a common intermediate of both pathways which must be cleaved by the MhpB dioxygenase before entering into the primary cell metabolism. Therefore, the degradation of PP has to be controlled by both its specific regulator (HcaR) but also by the MhpR regulator of the mhp cluster. We have demonstrated that 3HPP and DHPP are the true and best activators of MhpR, whereas PP only induces no response. However, in vivo and in vitro transcription experiments have demonstrated that PP activates the MhpR regulator synergistically with the true inducers, representing the first case of such a peculiar synergistic effect described for a bacterial regulator. The three compounds enhanced the interaction of MhpR with its DNA operator in electrophoretic mobility shift assays. Inducer binding to MhpR is detected by circular dichroism and fluorescence spectroscopies. Fluorescence quenching measurements have revealed that the true inducers (3HPP and DHPP) and PP bind with similar affinities and independently to MhpR. This type of dual-metabolite synergy provides great potential for a rapid modulation of gene expression and represents an important feature of transcriptional control. The mhp regulatory system is an example of the high complexity achievable in prokaryotes.

Toward the assessment of food toxicity for celiac patients: characterization of monoclonal antibodies to a main immunogenic gluten peptide

BACKGROUND AND AIMS

Celiac disease is a permanent intolerance to gluten prolamins from wheat, barley, rye and, in some patients, oats. Partially digested gluten peptides produced in the digestive tract cause inflammation of the small intestine. High throughput, immune-based assays using monoclonal antibodies specific for these immunotoxic peptides would facilitate their detection in food and enable monitoring of their enzymatic detoxification. Two monoclonal antibodies, G12 and A1, were developed against a highly immunotoxic 33-mer peptide. The potential of each antibody for quantifying food toxicity for celiac patients was studied.

METHODS

Epitope preferences of G12 and A1 antibodies were determined by ELISA with gluten-derived peptide variants of recombinant, synthetic or enzymatic origin.

RESULTS

The recognition sequences of G12 and A1 antibodies were hexameric and heptameric epitopes, respectively. Although G12 affinity for the 33-mer was superior to A1, the sensitivity for gluten detection was higher for A1. This observation correlated to the higher number of A1 epitopes found in prolamins than G12 epitopes. Activation of T cell from gluten digested by glutenases decreased equivalently to the detection of intact peptides by A1 antibody. Peptide recognition of A1 included gliadin peptides involved in the both the adaptive and innate immunological response in celiac disease.

CONCLUSIONS

The sensitivity and epitope preferences of the A1 antibody resulted to be useful to detect gluten relevant peptides to infer the potential toxicity of food for celiac patients as well as to monitor peptide modifications by transglutaminase 2 or glutenases.

Sensitive detection of cereal fractions that are toxic to celiac disease patients by using monoclonal antibodies to a main immunogenic wheat peptide

BACKGROUND

Celiac disease is an immune-mediated enteropathy caused by the ingestion of gluten, a protein fraction found in certain cereals. Immunotoxic gluten peptides that are recalcitrant to degradation of digestive enzymes appear to trigger celiac syndromes. A 33-mer peptide from alpha-2 gliadin has been identified as a principal contributor to gluten immunotoxicity. A gluten-free diet is the usual first therapy for celiac disease patients; therefore, the characterization and quantification of the toxic portion of the gluten in foodstuffs is crucial to avoid celiac damage.

OBJECTIVE

We aimed to develop immunologic assays as a novel food analysis tool for measuring cereal fractions that are immunotoxic to celiac disease patients.

DESIGN

The design focused on the production of monoclonal antibodies against the gliadin 33-mer peptide and the development of enzyme-linked immunosorbent assays (ELISAs) and Western blot analysis with the use of novel antibodies.

RESULTS

A sandwich ELISA method showed a detection limit for wheat, barley, and rye of <1 ppm prolamine. However, the method required a sample that was > or =1 order of magnitude greater for the detection of low-toxic oats, and there was no signal with the safe cereals maize and rice. A competitive ELISA method was also developed for detection of the toxic peptide in hydrolyzed food, which had a detection limit of <0.5 ppm gliadin.

CONCLUSIONS

Both ELISAs designed for use with the toxic gliadin 33-mer peptide suggested a high correlation between the presence of the peptide and the amount of cereal that was toxic to celiac disease patients. The sensitivity was significantly higher than that of equivalent methods recognizing other gluten epitopes.

In vivo drafting of single-chain antibodies for regulatory duty on the sigma54-promoter Pu of the TOL plasmid

The identification of single-chain antibodies (scFvs) that interfere in vivo with the building of the complex that activate the prokaryotic, sigma54-dependent promoter Pu of the catabolic TOL plasmid pWW0 is reported. To this end, a phage M13 library of scFvs was raised against the cognate prokaryotic enhancer-binding activator, XylR. The scFv pool was then expressed intracellularly in a reporter Pu-lacZ strain of Escherichia coli designed to permit formation of intramolecular disulphide bonds in cytoplasmic proteins. This strain allowed the assembly of functional scFvs and the direct testing of their activity on the Pu promoter in vivo. Specifically, genetic screening for lacZ-minus colonies yielded a number of scFvs able to downregulate transcriptional output in live cells. Two antibody clones were purified and shown to inhibit the activity of the same promoter in vitro as well. These scFvs targeted the DNA-binding domain of XylR and its ATP binding site respectively. This work provides a proof of principle that mimetic regulatory factors can be derived from an antibody repertoire that specifically interact with given transcriptional activators. As assembly of initiation complexes is stimulated or inhibited by regulatory proteins we argue that anti-XylR scFvs operate as bona fide transcriptional inhibitors of the Pu promoter.

Transcriptional regulators à la carte: engineering new effector specificities in bacterial regulatory proteins

For many regulators of bacterial biodegradation pathways, small molecule/effector binding is the signal for triggering transcriptional activation. Thus, regulation results from a cross-talk between chemicals sensed by transcriptional factors and operon expression status. These features can be utilised in the construction of biosensors for a wide range of target compounds as, in principle, any regulatory protein whose activity is modulated by binding to a small molecule can have its effector/inducer profile artificially altered. The cognate specificities of a number of regulatory proteins have been modified as an astute approach to developing, among others, bacterial biosensors for environmentally relevant compounds.

A new generation of vectors with increased induction ratios by overimposing a second regulatory level by attenuation

A major drawback of regulated gene expression from vectors bearing strong promoters is the associated high basal expression level. Simple regulatory systems have an intrinsic limitation in the range of induction, and attempts to mutate promoters to reduce basal expression usually result in concomitant reduction of induced levels. We have explored the possibility of reducing basal levels of gene expression while keeping induced levels intact by incorporating an additional regulatory circuit controlling a different step of the expression process. We have integrated the nasFEDCBA transcriptional attenuation system of Klebsiella oxytoca into a cascade expression circuit based on different regulatory elements of Pseudomonas putida, and also into a system based on the tac promoter, to expand their regulatory capacity. Basal expression from the promoters of these circuits was reduced by more than 10-fold by the nasF attenuator sequence while keeping the induced levels intact in the presence of the antiterminator protein, thus increasing the induction ratio by up to 1700-fold. In addition, using different combinations of regulatory elements and inducing conditions, we were able to obtain a broad range of expression levels. These vectors and the concept of their design will be very useful in regulating overproduction of heterologous proteins both at laboratory and industrial scales.

Laboratory evolution of catabolic enzymes and pathways

The laboratory evolution of environmentally relevant enzymes and proteins has resulted in the generation of optimized and stabilized enzymes, as well as enzymes with activity against new substrates. Numerous methods, including random mutagenesis, site-directed mutagenesis and DNA shuffling, have been widely used to generate variants of existing enzymes. These evolved catabolic enzymes have application for improving biodegradation pathways, generating engineered pathways for the degradation of particularly recalcitrant compounds, and for the development of biocatalytic processes to produce useful compounds. Regulatory proteins associated with catabolic pathways have been utilized to generate biosensors for the detection of bioavailable concentrations of environmentally relevant chemicals.

Stable long-term indigo production by overexpression of dioxygenase genes using a chromosomal integrated cascade expression circuit

In our laboratory we have analyzed different factors to maximize the yield in heterologous protein expression for long-term cultivation, by combination of an efficient cascade expression system and stable integration in the bacterial chromosome. In this work, we have explored this system for the production of indigo dye as a model for biotechnological production, by expressing in Escherichia coli the thnA1A2A3A4 genes from Sphingomonas macrogolitabida strain TFA, which encode the components of a tetralin dioxygenase activity. We compared Ptac, and the Pm-based cascade expression circuit in a multicopy plasmid and stably integrated into the bacterial chromosome. Plasmid-based expression systems resulted in instability of indigo production when serially diluted batch experiments were performed without a selective pressure. This problem was solved by integrating the expression module in the chromosome. Despite the gene dosage reduction, the synergic effect of the cascade expression system produced comparable expression to the dioxygenase activity in the plasmid configuration but could be stably maintained for at least 5 days. Here, we show that the cascade amplification circuit integrated in the chromosome could be an excellent system for tight control and stable production of recombinant products.

Illuminating the detection chain of bacterial bioreporters

Engineering bacteria for measuring chemicals of environmental or toxicological concern (bioreporter bacteria) has grown slowly into a mature research area. Despite many potential advantages, current bioreporters do not perform well enough to comply with environmental detection standards. Basically, the reasons for this are the lack of engineering principles in the detection chain in the bioreporters. Here, we dissect critical steps in the detection chain and illustrate how bioreporter design could be improved by mutagenizing specificity and selectivity of the sensing and regulatory proteins, by newer expression strategies and application of different signalling networks. Furthermore, we describe how redesigning bioreporter assays with respect to pollutant transport into the cells and application of other detection devices can decrease detection limits and increase the speed of detection.

Improvement of recombinant protein yield by a combination of transcriptional amplification and stabilization of gene expression

We explored the use of a cascade circuit for heterologous gene expression that consists of a regulatory module with a salicylate-inducible system that controls the expression of a second regulator, xylS2, whose product is activated by common inducers. Activation and increasing the concentration of the second regulator synergistically induced heterologous genes downstream of the Pm promoter in the expression module. This module can be placed in multicopy vectors or in the chromosome of a host strain by means of minitransposons. Using reporter genes, we evaluated gene regulation capacity and gross production of the system with different configurations. The highest yield was obtained when the expression module was in a multicopy plasmid after a 6-h induction. However, expression modules in plasmids showed low stability after induction even with selective pressure. The chromosomal configuration had the lowest basal levels and induced levels comparable to those of plasmid configurations, resulting in accumulation of more than 10% of the total protein. Unlike the configurations in plasmids, the yield was maintained for at least 3 days even without selective pressure. In conclusion, the cascade system in the chromosome configuration is more efficient for long-term fermentation because of the great stability of the overexpressing phenotype in spite of the high levels of expression.