Two-component signal transduction in Corynebacterium glutamicum and other corynebacteria: on the way towards stimuli and targets

The murein endopeptidase MepA regulated by MtrAB and MprAB participate in cell wall homeostasis

The complete genome of Corynebacterium glutamicum contain a gene encoding murein endopeptidase MepA which maintain cell wall homeostasis by regulating peptidoglycan biosynthesis. In this study, we investigate the physiological function, localization and regulator of MepA. The result shows that mepA overexpression lead to peptidoglycan degradation and the defects in cell division. MepA-EGFP was shown to localizes exclusively at the cell cell septum. In addition, mepA overexpression increased cell permeability and reduced the resistance of cells to isoniazid, an antibiotic used to treat Mycobacterium tuberculosis infection. Furthermore, transcription analysis showed that mepA affected cell division and membrane transport pathways, and was coordinately regulated by the two-component systems MtrAB and MprAB(CgtS/R2).

Revealing Corynebacterium glutamicum proteoforms through top-down proteomics

Corynebacterium glutamicum is a bacterium widely employed in the industrial production of amino acids as well as a broad range of other biotechnological products. The present study describes the characterization of C. glutamicum proteoforms, and their post-translational modifications (PTMs) employing top-down proteomics. Despite previous evidence of PTMs having roles in the regulation of C. glutamicum metabolism, this is the first top-down proteome analysis of this organism. We identified 1125 proteoforms from 273 proteins, with 60% of proteins presenting at least one mass shift, suggesting the presence of PTMs, including several acetylated, oxidized and formylated proteoforms. Furthermore, proteins relevant to amino acid production, protein secretion, and oxidative stress were identified with mass shifts suggesting the presence of uncharacterized PTMs and proteoforms that may affect biotechnologically relevant processes in this industrial workhorse. For instance, the membrane proteins mepB and SecG were identified as a cleaved and a formylated proteoform, respectively. While in the central metabolism, OdhI was identified as two proteoforms with potential biological relevance: a cleaved proteoform and a proteoform with PTMs corresponding to a 70 Da mass shift.

A manually curated compendium of expression profiles for the microbial cell factory Corynebacterium glutamicum

Corynebacterium glutamicum is the major host for the industrial production of amino acids and has become one of the best studied model organisms in microbial biotechnology. Rational strain construction has led to an improvement of producer strains and to a variety of novel producer strains with a broad substrate and product spectrum. A key factor for the success of these approaches is detailed knowledge of transcriptional regulation in C. glutamicum. Here, we present a large compendium of 927 manually curated microarray-based transcriptional profiles for wild-type and engineered strains detecting genome-wide expression changes of the 3,047 annotated genes in response to various environmental conditions or in response to genetic modifications. The replicates within the 927 experiments were combined to 304 microarray sets ordered into six categories that were used for differential gene expression analysis. Hierarchical clustering confirmed that no outliers were present in the sets. The compendium provides a valuable resource for future fundamental and applied research with C. glutamicum and contributes to a systemic understanding of this microbial cell factory.

Measurement(s)	Gene Expression Analysis
Technology Type(s)	Two Color Microarray
Factor Type(s)	WT condition A vs. WT condition B • Plasmid-based gene overexpression in parental strain vs. parental strain with empty vector control • Deletion mutant vs. parental strain
Sample Characteristic - Organism	Corynebacterium glutamicum
Sample Characteristic - Environment	laboratory environment
Sample Characteristic - Location	Germany

A pseudokinase version of the histidine kinase ChrS promotes high heme tolerance of Corynebacterium glutamicum

Heme is an essential cofactor for almost all living cells by acting as prosthetic group for various proteins or serving as alternative iron source. However, elevated levels are highly toxic for cells. Several corynebacterial species employ two paralogous, heme-responsive two-component systems (TCS), ChrSA and HrrSA, to cope with heme stress and to maintain intracellular heme homeostasis. Significant cross-talk at the level of phosphorylation between these systems was previously demonstrated. In this study, we have performed a laboratory evolution experiment to adapt Corynebacterium glutamicum to increasing heme levels. Isolated strains showed a highly increased tolerance to heme growing at concentrations of up to 100 μM. The strain featuring the highest heme tolerance harbored a frameshift mutation in the catalytical and ATPase-domain (CA-domain) of the chrS gene, converting it into a catalytically-inactive pseudokinase (ChrS_CA-fs). Reintroduction of the respective mutation in the parental C. glutamicum strain confirmed high heme tolerance and showed a drastic upregulation of hrtBA encoding a heme export system, conserved in Firmicutes and Actinobacteria. The strain encoding the ChrS pseudokinase variant showed significantly higher heme tolerance than a strain lacking chrS. Mutational analysis revealed that induction of hrtBA in the evolved strain is solely mediated via the cross-phosphorylation of the response regulator (RR) ChrA by the kinase HrrS and BACTH assays revealed the formation of heterodimers between HrrS and ChrS. Overall, our results emphasize an important role of the ChrS pseudokinase in high heme tolerance of the evolved C. glutamicum and demonstrate the promiscuity in heme-dependent signaling of the paralogous two-component systems facilitating fast adaptation to changing environmental conditions.

Transcriptome profiles of high-lysine adaptation reveal insights into osmotic stress response in Corynebacterium glutamicum

Corynebacterium glutamicum has been widely and effectively used for fermentative production of l-lysine on an industrial scale. However, high-level accumulation of end products inevitably leads to osmotic stress and hinders further increase of l-lysine production. At present, the underlying mechanism by which C. glutamicum cells adapt to high-lysine-induced osmotic stress is still unclear. In this study, we conducted a comparative transcriptomic analysis by RNA-seq to determine gene expression profiles under different high-lysine stress conditions. The results indicated that the increased expression of some metabolic pathways such as sulfur metabolism and specific amino acid biosynthesis might offer favorable benefits for high-lysine adaptation. Functional assays of 18 representative differentially expressed genes showed that the enhanced expression of multiple candidate genes, especially grpE chaperon, conferred high-lysine stress tolerance in C. glutamicum. Moreover, DNA repair component MutT and energy-transducing NADH dehydrogenase Ndh were also found to be important for protecting cells against high-lysine-induced osmotic stress. Taken together, these aforementioned findings provide broader views of transcriptome profiles and promising candidate targets of C. glutamicum for the adaptation of high-lysine stress during fermentation.

The diversity of heme sensor systems - heme-responsive transcriptional regulation mediated by transient heme protein interactions

Heme is a versatile molecule that is vital for nearly all cellular life by serving as prosthetic group for various enzymes or as nutritional iron source for diverse microbial species. However, elevated levels of heme molecule are toxic to cells. The complexity of this stimulus has shaped the evolution of diverse heme sensor systems, which are involved in heme-dependent transcriptional regulation in eukaryotes and prokaryotes. The functions of these systems are manifold - ranging from the specific control of heme detoxification or uptake systems to the global integration of heme and iron homeostasis. This review focuses on heme sensor systems, regulating heme homeostasis by transient heme protein interaction. We provide an overview of known heme-binding motifs in prokaryotic and eukaryotic transcription factors. Besides the central ligands, the surrounding amino acid environment was shown to play a pivotal role in heme binding. The diversity of heme-regulatory systems therefore illustrates that prediction based on pure sequence information is hardly possible and requires careful experimental validation. Comprehensive understanding of heme-regulated processes is not only important for our understanding of cellular physiology, but also provides a basis for the development of novel antibacterial drugs and metabolic engineering strategies.

Overlapping SigH and SigE sigma factor regulons in Corynebacterium glutamicum

The sigma H (σ^Η) and sigma E (σ^E) subunits of Corynebacterium glutamicum RNA polymerase belong to Group 4 of sigma factors, also called extracytoplasmic function (ECF) sigma factors. Genes of the C. glutamicum σ^Η regulon that are involved in heat and oxidative stress response have already been defined, whereas the genes of the σ^E regulon, which is involved in cell surface stress response, have not been explored until now. Using the C. glutamicum RES167 strain and its derivative C. glutamicum ΔcseE with a deletion in the anti-σ^Ε gene, differential gene expression was analyzed by RNA sequencing. We found 296 upregulated and 398 downregulated genes in C. glutamicum ΔcseE compared to C. glutamicum RES167. To confirm the functional link between σ^Ε and the corresponding promoters, we tested selected promoters using the in vivo two-plasmid system with gfpuv as a reporter gene and by in vitro transcription. Analyses with RNAP+σ^Η and RNAP+σ^Ε, which were previously shown to recognize similar promoters, proved that the σ^Η and σ^E regulons significantly overlap. The σ^E-controlled genes were found to be involved for example in protein quality control (dnaK, dnaJ2, clpB, and clpC), the regulation of Clp proteases (clgR), and membrane integrity maintenance. The single-promoter analyses with σ^Η and σ^Ε revealed that there are two groups of promoters: those which are exclusively σ^Η-specific, and the other group of promoters, which are σ^Η/σ^E-dependent. No exclusively σ^E-dependent promoter was detected. We defined the consensus sequences of exclusively σ^Η-regulated promotors to be -35 GGAAt and - 10 GTT and σ^Η/σ^E-regulated promoters to be -35 GGAAC and - 10 cGTT. Fifteen genes were found to belong to the σ^Η/σ^Ε regulon. Homology modeling showed that there is a specific interaction between Met170 in σ^Η and the nucleotides -31 and - 30 within the non-coding strand (AT or CT) of the σ^Η-dependent promoters. In σ^E, Arg185 was found to interact with the nucleotides GA at the same positions in the σ^E-dependent promoters.

Development of a Hyperosmotic Stress Inducible Gene Expression System by Engineering the MtrA/MtrB-Dependent NCgl1418 Promoter in Corynebacterium glutamicum

Corynebacterium glutamicum is an important workhorse for industrial production of diversiform bioproducts. Precise regulation of gene expression is crucial for metabolic balance and enhancing production of target molecules. Auto-inducible promoters, which can be activated without expensive inducers, are ideal regulatory tools for industrial-scale application. However, few auto-inducible promoters have been identified and applied in C. glutamicum. Here, a hyperosmotic stress inducible gene expression system was developed and used for metabolic engineering of C. glutamicum. The promoter of NCgl1418 (P _NCgl1418 ) that was activated by the two-component signal transduction system MtrA/MtrB was found to exhibit a high inducibility under hyperosmotic stress conditions. A synthetic promoter library was then constructed by randomizing the flanking and space regions of P _NCgl1418 , and mutant promoters exhibiting high strength were isolated via fluorescence activated cell sorting (FACS)-based high-throughput screening. The hyperosmotic stress inducible gene expression system was applied to regulate the expression of lysE encoding a lysine exporter and repress four genes involved in lysine biosynthesis (gltA, pck, pgi, and hom) by CRISPR interference, which increased the lysine titer by 64.7% (from 17.0 to 28.0 g/L) in bioreactors. The hyperosmotic stress inducible gene expression system developed here is a simple and effective tool for gene auto-regulation in C. glutamicum and holds promise for metabolic engineering of C. glutamicum to produce valuable chemicals and fuels.

Corynebacterium glutamicum Regulation beyond Transcription: Organizing Principles and Reconstruction of an Extended Regulatory Network Incorporating Regulations Mediated by Small RNA and Protein-Protein Interactions

Corynebacterium glutamicum is a Gram-positive bacterium found in soil where the condition changes demand plasticity of the regulatory machinery. The study of such machinery at the global scale has been challenged by the lack of data integration. Here, we report three regulatory network models for C. glutamicum: strong (3040 interactions) constructed solely with regulations previously supported by directed experiments; all evidence (4665 interactions) containing the strong network, regulations previously supported by nondirected experiments, and protein-protein interactions with a direct effect on gene transcription; sRNA (5222 interactions) containing the all evidence network and sRNA-mediated regulations. Compared to the previous version (2018), the strong and all evidence networks increased by 75 and 1225 interactions, respectively. We analyzed the system-level components of the three networks to identify how they differ and compared their structures against those for the networks of more than 40 species. The inclusion of the sRNA-mediated regulations changed the proportions of the system-level components and increased the number of modules but decreased their size. The C. glutamicum regulatory structure contrasted with other bacterial regulatory networks. Finally, we used the strong networks of three model organisms to provide insights and future directions of the C.glutamicum regulatory network characterization.

Corynebacterium diphtheriae Proteome Adaptation to Cell Culture Medium and Serum

Host-pathogen interactions are often studied in vitro using primary or immortal cell lines. This set-up avoids ethical problems of animal testing and has the additional advantage of lower costs. However, the influence of cell culture media on bacterial growth and metabolism is not considered or investigated in most cases. To address this question growth and proteome adaptation of Corynebacterium diphtheriae strain ISS3319 were investigated in this study. Bacteria were cultured in standard growth medium, cell culture medium, and fetal calf serum. Mass spectrometric analyses and label-free protein quantification hint at an increased bacterial pathogenicity when grown in cell culture medium as well as an influence of the growth medium on the cell envelope.

A universal stress protein in Mycobacterium smegmatis sequesters the cAMP-regulated lysine acyltransferase and is essential for biofilm formation

Universal stress proteins (USPs) are present in many bacteria, and their expression is enhanced under various environmental stresses. We have previously identified a USP in Mycobacterium smegmatis that is a product of the msmeg_4207 gene and is a substrate for a cAMP-regulated protein lysine acyltransferase (KATms; MSMEG_5458). Here, we explored the role of this USP (USP4207) in M. smegmatis and found that its gene is present in an operon that also contains genes predicted to encode a putative tripartite tricarboxylate transporter (TTT). Transcription of the TTT-usp4207 operon was induced in the presence of citrate and tartrate, perhaps by the activity of a divergent histidine kinase-response regulator gene pair. A usp4207-deleted strain had rough colony morphology and reduced biofilm formation compared with the WT strain; however, both normal colony morphology and biofilm formation were restored in a Δusp4207Δkatms strain. We identified several proteins whose acetylation was lost in the Δkatms strain, and whose transcript levels increased in M. smegmatis biofilms along with that of USP4207, suggesting that USP4207 insulates KATms from its other substrates in the cell. We propose that USP4207 sequesters KATms from diverse substrates whose activities are down-regulated by acylation but are required for biofilm formation, thus providing a defined role for this USP in mycobacterial physiology and stress responses.

Responses of Microorganisms to Osmotic Stress

The cytoplasm of bacterial cells is a highly crowded cellular compartment that possesses considerable osmotic potential. As a result, and owing to the semipermeable nature of the cytoplasmic membrane and the semielastic properties of the cell wall, osmotically driven water influx will generate turgor, a hydrostatic pressure considered critical for growth and viability. Both increases and decreases in the external osmolarity inevitably trigger water fluxes across the cytoplasmic membrane, thus impinging on the degree of cellular hydration, molecular crowding, magnitude of turgor, and cellular integrity. Here, we assess mechanisms that permit the perception of osmotic stress by bacterial cells and provide an overview of the systems that allow them to genetically and physiologically cope with this ubiquitous environmental cue. We highlight recent developments implicating the secondary messenger c-di-AMP in cellular adjustment to osmotic stress and the role of osmotic forces in the life of bacteria-assembled in biofilms.

Toxic but tasty - temporal dynamics and network architecture of heme-responsive two-component signaling in Corynebacterium glutamicum

Heme is an essential cofactor and alternative iron source for almost all bacterial species but may cause severe toxicity upon elevated levels and consequently, regulatory mechanisms coordinating heme homeostasis represent an important fitness trait. A remarkable scenario is found in several corynebacterial species, e.g. Corynebacterium glutamicum and Corynebacterium diphtheriae, which dedicate two paralogous, heme-responsive two-component systems, HrrSA and ChrSA, to cope with the Janus nature of heme. Here, we combined experimental reporter profiling with a quantitative mathematical model to understand how this particular regulatory network architecture shapes the dynamic response to heme. Our data revealed an instantaneous activation of the detoxification response (hrtBA) upon stimulus perception and we found that kinase activity of both kinases contribute to this fast onset. Furthermore, instant deactivation of the P_hrtBA promoter is achieved by a strong ChrS phosphatase activity upon stimulus decline. While the activation of detoxification response is uncoupled from further factors, heme utilization is additionally governed by the global iron regulator DtxR integrating information on iron availability into the regulatory network. Altogether, our data provide comprehensive insights how TCS cross-regulation and network hierarchy shape the temporal dynamics of detoxification (hrtBA) and utilization (hmuO) as part of a global homeostatic response to heme.

Triple deletion of clpC, porB, and mepA enhances production of small ubiquitin-like modifier-N-terminal pro-brain natriuretic peptide in Corynebacterium glutamicum

In our previous work, a two-plasmid CRISPR/Cas9 system was constructed for genome editing in Corynebacterium glutamicum. To increase the transformation efficiency and simplify the plasmid curing steps, an all-in-one CRISPR/Cas9 system was constructed for efficient genome editing. In addition, to research proteolysis during the production of recombinant proteins and generate a host for enhanced expression of recombinant proteins, the system was used to delete three genes, clpC, porB, and mepA in C. glutamicum CGMCC1.15647, which encoded the Clp protease subunit ClpC, anion selective channel protein B, and metallopeptidase A, respectively. After the evaluation of different plasmids and hosts, small ubiquitin-like modifier-N-terminal pro-brain natriuretic peptide (SUMO-NT-proBNP), an important protein used for the diagnosis of mild heart failure was successfully expressed in the triple mutant ΔclpCΔporBΔmepA, which exhibit threefold higher levels of protein expression compared with the wild-type. In conclusion, we created a simplified CRISPR tool for genome editing in C. glutamicum, provided a method to generate a host for enhanced expression of recombinant proteins and successfully expressed SUMO-NT-proBNP in C. glutamicum. This tool and method will greatly facilitate genetic engineering and metabolic optimization of this important platform organism.

Biosynthesis of organic photosensitizer Zn-porphyrin by diphtheria toxin repressor (DtxR)-mediated global upregulation of engineered heme biosynthesis pathway in Corynebacterium glutamicum

Zn-porphyrin is a promising organic photosensitizer in various fields including solar cells, interface and biomedical research, but the biosynthesis study has been limited, probably due to the difficulty of understanding complex biosynthesis pathways. In this study, we developed a Corynebacterium glutamicum platform strain for the biosynthesis of Zn-coproporphyrin III (Zn-CP III), in which the heme biosynthesis pathway was efficiently upregulated. The pathway was activated and reinforced by strong promoter-induced expression of hemA^M (encoding mutated glutamyl-tRNA reductase) and hemL (encoding glutamate-1-semialdehyde aminotransferase) genes. This engineered strain produced 33.54 ± 3.44 mg/l of Zn-CP III, while the control strain produced none. For efficient global regulation of the complex pathway, the dtxR gene encoding the transcriptional regulator diphtheria toxin repressor (DtxR) was first overexpressed in C. glutamicum with hemA^M and hemL genes, and its combinatorial expression was improved by using effective genetic tools. This engineered strain biosynthesized 68.31 ± 2.15 mg/l of Zn-CP III. Finally, fed-batch fermentation allowed for the production of 132.09 mg/l of Zn-CP III. This titer represents the highest in bacterial production of Zn-CP III reported to date, to our knowledge. This study demonstrates that engineered C. glutamicum can be a robust biotechnological model for the production of photosensitizer Zn-porphyrin.

Population heterogeneity in microbial bioprocesses: origin, analysis, mechanisms, and future perspectives

Population heterogeneity is omnipresent in all bioprocesses even in homogenous environments. Its origin, however, is only so well understood that potential strategies like bet-hedging, noise in gene expression and division of labour that lead to population heterogeneity can be derived from experimental studies simulating the dynamics in industrial scale bioprocesses. This review aims at summarizing the current state of the different parts of single cell studies in bioprocesses. This includes setups to visualize different phenotypes of single cells, computational approaches connecting single cell physiology with environmental influence and special cultivation setups like scale-down reactors that have been proven to be useful to simulate large-scale conditions. A step in between investigation of populations and single cells is studying subpopulations with distinct properties that differ from the rest of the population with sub-omics methods which are also presented here. Moreover, the current knowledge about population heterogeneity in bioprocesses is summarized for relevant industrial production hosts and mixed cultures, as they provide the unique opportunity to distribute metabolic burden and optimize production processes in a way that is impossible in traditional monocultures. In the end, approaches to explain the underlying mechanism of population heterogeneity and the evidences found to support each hypothesis are presented. For instance, population heterogeneity serving as a bet-hedging strategy that is used as coordinated action against bioprocess-related stresses while at the same time spreading the risk between individual cells as it ensures the survival of least a part of the population in any environment the cells encounter.

Membrane Topology and Heme Binding of the Histidine Kinases HrrS and ChrS in Corynebacterium glutamicum

The HrrSA and the ChrSA two-component systems play a central role in the coordination of heme homeostasis in the Gram-positive soil bacterium Corynebacterium glutamicum and the prominent pathogen Corynebacterium diphtheriae, both members of the Corynebacteriaceae. In this study, we have performed a comparative analysis of the membrane topology and heme-binding characteristics of the histidine kinases HrrS and ChrS of C. glutamicum. While the cytoplasmic catalytic domains are highly conserved between HrrS and ChrS, the N-terminal sensing parts share only minor sequence similarity. PhoA and LacZ fusions of the N-terminal sensor domains of HrrS and ChrS revealed that both proteins are embedded into the cytoplasmic membrane via six α-helices. Although the overall membrane topology appeared to be conserved, target gene profiling indicated a higher sensitivity of the ChrS system to low heme levels (< 1 μM). In vitro, solubilized and purified full-length proteins bound heme in a 1:1 stoichiometry per monomer. Alanine-scanning of conserved amino acid residues in the N-terminal sensor domain revealed three aromatic residues (Y¹¹², F¹¹⁵, and F¹¹⁸), which apparently contribute to heme binding of HrrS. Exchange of either one or all three residues resulted in an almost abolished heme binding of HrrS in vitro. In contrast, ChrS mutants only displayed a red shift of the soret band from 406 to 418 nm suggesting an altered set of ligands in the triple mutant. In line with target gene profiling, these in vitro studies suggest distinct differences in the heme-protein interface of HrrS and ChrS. Since the membrane topology mapping displayed no extensive loop regions and alanine-scanning revealed potential heme-binding residues in α-helix number four, we propose an intramembrane sensing mechanism for both proteins. Overall, we present a first comparative analysis of the ChrS and HrrS kinases functioning as transient heme sensors in the Corynebacteriaceae.

Efficient gene editing in Corynebacterium glutamicum using the CRISPR/Cas9 system

BACKGROUND

Corynebacterium glutamicum (C. glutamicum) has traditionally been used as a microbial cell factory for the industrial production of many amino acids and other industrially important commodities. C. glutamicum has recently been established as a host for recombinant protein expression; however, some intrinsic disadvantages could be improved by genetic modification. Gene editing techniques, such as deletion, insertion, or replacement, are important tools for modifying chromosomes.

RESULTS

In this research, we report a CRISPR/Cas9 system in C. glutamicum for rapid and efficient genome editing, including gene deletion and insertion. The system consists of two plasmids: one containing a target-specific guide RNA and a homologous sequence to a target gene, the other expressing Cas9 protein. With high efficiency (up to 100%), this system was used to disrupt the porB, mepA, clpX and Ncgl0911 genes, which affect the ability to express proteins. The porB- and mepA-deletion strains had enhanced expression of green fluorescent protein, compared with the wild-type stain. This system can also be used to engineer point mutations and gene insertions.

CONCLUSIONS

In this study, we adapted the CRISPR/Cas9 system from S. pyogens to gene deletion, point mutations and insertion in C. glutamicum. Compared with published genome modification methods, methods based on the CRISPR/Cas9 system can rapidly and efficiently achieve genome editing. Our research provides a powerful tool for facilitating the study of gene function, metabolic pathways, and enhanced productivity in C. glutamicum.

The three-component system EsrISR regulates a cell envelope stress response in Corynebacterium glutamicum

When the cell envelope integrity is compromised, bacteria trigger signaling cascades resulting in the production of proteins that counteract these extracytoplasmic stresses. Here, we show that the two-component system EsrSR regulates a cell envelope stress response in the Actinobacterium Corynebacterium glutamicum. The sensor kinase EsrS possesses an amino-terminal phage shock protein C (PspC) domain, a property that sets EsrSR apart from all other two-component systems characterized so far. An integral membrane protein, EsrI, whose gene is divergently transcribed to the esrSR gene locus and which interestingly also possesses a PspC domain, acts as an inhibitor of EsrSR under non-stress conditions. The resulting EsrISR three-component system is activated among others by antibiotics inhibiting the lipid II cycle, such as bacitracin and vancomycin, and it orchestrates a broad regulon including the esrI-esrSR gene locus itself, genes encoding heat shock proteins, ABC transporters, and several putative membrane-associated or secreted proteins of unknown function. Among those, the ABC transporter encoded by cg3322-3320 was shown to be directly involved in bacitracin resistance of C. glutamicum. Since similar esrI-esrSR loci are present in a large number of actinobacterial genomes, EsrISR represents a novel type of stress-responsive system whose components are highly conserved in the phylum Actinobacteria.

A Toolbox of Genetically Encoded FRET-Based Biosensors for Rapid l-Lysine Analysis

Background: The fast development of microbial production strains for basic and fine chemicals is increasingly carried out in small scale cultivation systems to allow for higher throughput. Such parallelized systems create a need for new rapid online detection systems to quantify the respective target compound. In this regard, biosensors, especially genetically encoded Förster resonance energy transfer (FRET)-based biosensors, offer tremendous opportunities. As a proof-of-concept, we have created a toolbox of FRET-based biosensors for the ratiometric determination of l-lysine in fermentation broth. Methods: The sensor toolbox was constructed based on a sensor that consists of an optimized central lysine-/arginine-/ornithine-binding protein (LAO-BP) flanked by two fluorescent proteins (enhanced cyan fluorescent protein (ECFP), Citrine). Further sensor variants with altered affinity and sensitivity were obtained by circular permutation of the binding protein as well as the introduction of flexible and rigid linkers between the fluorescent proteins and the LAO-BP, respectively. Results: The sensor prototype was applied to monitor the extracellular l-lysine concentration of the l-lysine producing Corynebacterium glutamicum (C. glutamicum) strain DM1933 in a BioLector^® microscale cultivation device. The results matched well with data obtained by HPLC analysis and the Ninhydrin assay, demonstrating the high potential of FRET-based biosensors for high-throughput microbial bioprocess optimization.

Transcriptomic analysis for elucidating the physiological effects of 5-aminolevulinic acid accumulation on Corynebacterium glutamicum

5-Aminolevulinic acid (ALA), the committed intermediate of the heme biosynthetic pathway, attracts close attention among researchers because of its potential applications to cancer treatment and agriculture. Overexpression of heterologous hemA and hemL, which encode glutamyl-tRNA reductase and glutamate-1-semialdehyde aminotransferase, respectively, in Corynebacterium glutamicum produces ALA, although whether ALA accumulation causes unintended effects on the host is unknown. Here we used an integrated systems approach to compare global transcriptional changes induced by the expression of hemA and hemL. Metabolic pathway such as glycolysis was inhibited, but tricarboxylic acid cycle, pentose phosphate pathway, and respiratory metabolism were stimulated. Moreover, the transcriptional levels of certain genes involved in heme biosynthesis were up-regulated, and the data implicate the two-component system (TCS) HrrSA was involved in the regulation of heme synthesis. With these understandings, it is proposed that ALA accumulation stimulates heme synthesis pathway and respiratory metabolism. Our study illuminates the physiological effects of overexpressing hemA and hemL on the phenotype of C. glutamicum and contributes important insights into the regulatory mechanisms of the heme biosynthetic pathways.

Phosphatase activity of the histidine kinases ensures pathway specificity of the ChrSA and HrrSA two-component systems in Corynebacterium glutamicum

The majority of bacterial genomes encode a high number of two-component systems controlling gene expression in response to a variety of different stimuli. The Gram-positive soil bacterium Corynebacterium glutamicum contains two homologous two-component systems (TCS) involved in the haem-dependent regulation of gene expression. Whereas the HrrSA system is crucial for utilization of haem as an alternative iron source, ChrSA is required to cope with high toxic haem levels. In this study, we analysed the interaction of HrrSA and ChrSA in C. glutamicum. Growth of TCS mutant strains, in vitro phosphorylation assays and promoter assays of P(hrtBA) and P(hmuO) fused to eyfp revealed cross-talk between both systems. Our studies further indicated that both kinases exhibit a dual function as kinase and phosphatase. Mutation of the conserved glutamine residue in the putative phosphatase motif DxxxQ of HrrS and ChrS resulted in a significantly increased activity of their respective target promoters (P(hmuO) and P(hrtBA) respectively). Remarkably, phosphatase activity of both kinases was shown to be specific only for their cognate response regulators. Altogether our data suggest the phosphatase activity of HrrS and ChrS as key mechanism to ensure pathway specificity and insulation of these two homologous systems.

Metabolic Regulation of a Bacterial Cell System with Emphasis on Escherichia coli Metabolism

It is quite important to understand the overall metabolic regulation mechanism of bacterial cells such as Escherichia coli from both science (such as biochemistry) and engineering (such as metabolic engineering) points of view. Here, an attempt was made to clarify the overall metabolic regulation mechanism by focusing on the roles of global regulators which detect the culture or growth condition and manipulate a set of metabolic pathways by modulating the related gene expressions. For this, it was considered how the cell responds to a variety of culture environments such as carbon (catabolite regulation), nitrogen, and phosphate limitations, as well as the effects of oxygen level, pH (acid shock), temperature (heat shock), and nutrient starvation.

The two-component system ChrSA is crucial for haem tolerance and interferes with HrrSA in haem-dependent gene regulation in Corynebacterium glutamicum

We recently showed that the two-component system (TCS) HrrSA plays a central role in the control of haem homeostasis in the Gram-positive soil bacterium Corynebacterium glutamicum. Here, we characterized the function of another TCS of this organism, ChrSA, which exhibits significant sequence similarity to HrrSA, and provide evidence for cross-regulation of the two systems. In this study, ChrSA was shown to be crucial for haem resistance of C. glutamicum by activation of the putative haem-detoxifying ABC-transporter HrtBA in the presence of haem. Deletion of either hrtBA or chrSA resulted in a strongly increased sensitivity towards haem. DNA microarray analysis and gel retardation assays with the purified response regulator ChrA revealed that phosphorylated ChrA acts as an activator of hrtBA in the presence of haem. The haem oxygenase gene, hmuO, showed a decreased mRNA level in a chrSA deletion mutant but no significant binding of ChrA to the hmuO promoter was observed in vitro. In contrast, activation from P(hmuO) fused to eyfp was almost abolished in an hrrSA mutant, indicating that HrrSA is the dominant system for haem-dependent activation of hmuO in C. glutamicum. Remarkably, ChrA was also shown to bind to the hrrA promoter and to repress transcription of the paralogous response regulator, whereas chrSA itself seemed to be repressed by HrrA. These data suggest a close interplay of HrrSA and ChrSA at the level of transcription and emphasize ChrSA as a second TCS involved in haem-dependent gene regulation in C. glutamicum, besides HrrSA.