Regulation of Aerobic Succinate Transporter dctA of E. coli by cAMP-CRP, DcuS-DcuR, and EIIAGlc: Succinate as a Carbon Substrate and Signaling Molecule.

INTRODUCTION: C4-dicarboxylates (C4-DC) have emerged as significant growth substrates and signaling molecules for various Enterobacteriaceae during their colonization of mammalian hosts. Particularly noteworthy is the essential role of fumarate respiration during colonization of pathogenic bacteria. To investigate the regulation of aerobic C4-DC metabolism, the study explored the transcriptional control of the main aerobic C4-DC transporter, dctA, under different carbohydrate conditions. In addition, mutants related to carbon catabolite repression (CCR) and C4-DC regulation (DcuS-DcuR) were examined to better understand the regulatory integration of aerobic C4-DC metabolism into CCR. For initial insight into posttranslational regulation, the interaction between the aerobic C4-DC transporter DctA and EIIAGlc from the glucose-specific phosphotransferase system was investigated. METHODS: The expression of dctA was characterized in the presence of various carbohydrates and regulatory mutants affecting CCR. This was accomplished by fusing the dctA promoter (PdctA) to the lacZ reporter gene. Additionally, the interaction between DctA and EIIAGlc of the glucose-specific phosphotransferase system was examined in vivo using a bacterial two-hybrid system. RESULTS: The dctA promoter region contains a class I cAMP-CRP-binding site at position -81.5 and a DcuR-binding site at position -105.5. DcuR, the response regulator of the C4-DC-activated DcuS-DcuR two-component system, and cAMP-CRP stimulate dctA expression. The expression of dctA is subject to the influence of various carbohydrates via cAMP-CRP, which differently modulate cAMP levels. Here we show that EIIAGlc of the glucose-specific phosphotransferase system strongly interacts with DctA, potentially resulting in the exclusion of C4-DCs when preferred carbon substrates, such as sugars, are present. In contrast to the classical inducer exclusion known for lactose permease LacY, inhibition of C4-DC uptake into the cytoplasm affects only its role as a substrate, but not as an inducer since DcuS detects C4-DCs in the periplasmic space ("substrate exclusion"). The work shows an interplay between cAMP-CRP and the DcuS-DcuR regulatory system for the regulation of dctA at both transcriptional and posttranslational levels. CONCLUSION: The study highlights a hierarchical interplay between global (cAMP-CRP) and specific (DcuS-DcuR) regulation of dctA at the transcriptional and posttranslational levels. The integration of global and specific transcriptional regulation of dctA, along with the influence of EIIAGlc on DctA, fine-tunes C4-DC catabolism in response to the availability of other preferred carbon sources. It attributes DctA a central role in the control of aerobic C4-DC catabolism and suggests a new role to EIIAGlc on transporters (control of substrate uptake by substrate exclusion).

Preliminary study on the effect of catabolite repression gene knockout on p-nitrophenol degradation in Pseudomonas putida DLL-E4.

P-nitrophenol (PNP) is a carcinogenic, teratogenic, and mutagenic compound that can cause serious harm to the environment. A strain of Pseudomonas putida DLL-E4, can efficiently degrade PNP in a complex process that is influenced by many factors. Previous studies showed that the expression level of pnpA, a key gene involved in PNP degradation, was upregulated significantly and the degradation of PNP was obviously accelerated in the presence of glucose. In addition, the expression of crc, crcY, and crcZ, key genes involved in catabolite repression, was downregulated, upregulated, and upregulated, respectively. To investigate the effect of the carbon catabolite repression (CCR) system on PNP degradation, the crc, crcY, and crcZ genes were successfully knocked out by conjugation experiments. Our results showed that the knockout of crc accelerated PNP degradation but slowed down the cell growth. However, the knockout of crcY or crcZ alone accelerated PNP degradation when PNP as the sole carbon source, but that knockout slowed down PNP degradation when glucose was added. The results indicate that the CCR system is involved in the regulation of PNP degradation, and further work is required to determine the details of the specific regulatory mechanism.

Genome-edited Saccharomyces cerevisiae strains for improving quality, safety, and flavor of fermented foods.

While Cas9-based genome editing enabled precise and sophisticated genetic perturbations in conventional and non-conventional yeast strains, its applications for food fermentations have been extremely limited. In order to improve quality and flavor of various yeast-fermented foods, we isolated and engineered a diploid or polyploid Saccharomyces cerevisiae strain (N1) which exhibits robust sugar fermentation, strong acid tolerance, and rapid gas production from Korean Nuruk. First, RGT2 and SNF3 coding for glucose sensors were deleted to increase respiration. A bread dough fermented with the N1ΔRGT2ΔSNF3 strain showed an 18% increased volume due to higher carbon dioxide production. Second, ASP3 coding for asparaginase was overexpressed and URE2 coding for a transcriptional factor of nitrogen catabolite repression (NCR) was deleted to increase asparagine consumption. When the N1ΔURE2::PGPD-ASP3 strain was applied to a potato dough, asparagine was rapidly depleted in the dough, resulting in potato chips with negligible amounts of acrylamide. Third, the N1ΔURE2 strain was utilized to increase levels of the amino acids which provide a savory taste during rice wine fermentation. The above genome-edited yeast strains contain no heterologous DNA. As such, they can be used to improve fermented foods with no subjection to GM regulation.

Overview of protein phosphorylation in bacteria with a main focus on unusual protein kinases in Bacillus subtilis.

Protein phosphorylation is a post-translational modification that affects protein activity through the addition of a phosphate moiety by protein kinases or phosphotransferases. It occurs in all life forms. In addition to Hanks kinases found also in eukaryotes, bacteria encode membrane histidine kinases that, with their cognate response regulator, constitute two-component systems and phosphotransferases that phosphorylate proteins involved in sugar utilization on histidine and cysteine residues. In addition, they encode BY-kinases and arginine kinases that phosphorylate protein specifically on tyrosine and arginine residues respectively. They also possess unusual bacterial protein kinases illustrated here by examples from Bacillus subtilis.

Dissecting the role of the two Streptomyces peucetius var. caesius glucokinases in the sensitivity to carbon catabolite repression.

Streptomyces peucetius var. caesius, the doxorubicin-producing strain, has two glucokinases (Glks) for glucose phosphorylation. One of them (ATP-Glk) uses adenosine triphosphate as its phosphate source, and the other one uses polyphosphate (PP). Glk regulates the carbon catabolite repression (CCR) process, as well as glucose utilization. However, in the streptomycetes, the specific role of each one of the Glks in these processes is unknown. With the use of PP- and ATP-Glk null mutants, we aimed to establish their respective role in glucose metabolism and their possible implication in the CCR. Our results supported that in S. peucetius var. caesius, both Glks allowed this strain to grow in different glucose concentrations. PP-Glk seems to be the main enzyme for glucose metabolism, and ATP-Glk is the only one involved in the CCR process affecting the levels of α-amylase and anthracycline production. Besides, analysis of Glk activities in the parental strain and the mutants revealed ATP-Glk as an enzyme negatively affected by high glucose concentrations. Although ATP-Glk utilizes only ATP as the substrate for glucose phosphorylation, probably PP-Glk can use either ATP or polyphosphate. Finally, a possible connection between both Glks may exist from the regulatory point of view.

Quantifying tagged mRNA export flux via nuclear pore complexes in single live cells.

The mRNA export flux through nuclear pore complexes (NPC) changes under DNA manipulation and hence affects protein translation. However, monitoring the flux of a specific mRNA in single live cell is beyond reach of traditional techniques. We developed a fluorescence-based detection method for measuring the export flux of mRNA through NPC in single live cell using a snapshot image, which had been tested on exogenous genes' expression in HeLa cells, with transfection or infection, and endogenous genes' expression in yeast cells, during incubation and carbon catabolite repression. With its speediness, explicitness and noninvasiveness, we believe that it would be valuable in direct monitoring of gene behavior, and the understanding of gene regulation at a single cell level.

The High Osmolarity Glycerol Mitogen-Activated Protein Kinase regulates glucose catabolite repression in filamentous fungi.

The utilization of different carbon sources in filamentous fungi underlies a complex regulatory network governed by signaling events of different protein kinase pathways, including the high osmolarity glycerol (HOG) and protein kinase A (PKA) pathways. This work unraveled cross-talk events between these pathways in governing the utilization of preferred (glucose) and non-preferred (xylan, xylose) carbon sources in the reference fungus Aspergillus nidulans. An initial screening of a library of 103 non-essential protein kinase (NPK) deletion strains identified several mitogen-activated protein kinases (MAPKs) to be important for carbon catabolite repression (CCR). We selected the MAPKs Ste7, MpkB, and PbsA for further characterization and show that they are pivotal for HOG pathway activation, PKA activity, CCR via regulation of CreA cellular localization and protein accumulation, as well as for hydrolytic enzyme secretion. Protein-protein interaction studies show that Ste7, MpkB, and PbsA are part of the same protein complex that regulates CreA cellular localization in the presence of xylan and that this complex dissociates upon the addition of glucose, thus allowing CCR to proceed. Glycogen synthase kinase (GSK) A was also identified as part of this protein complex and shown to potentially phosphorylate two serine residues of the HOG MAPKK PbsA. This work shows that carbon source utilization is subject to cross-talk regulation by protein kinases of different signaling pathways. Furthermore, this study provides a model where the correct integration of PKA, HOG, and GSK signaling events are required for the utilization of different carbon sources.

Regulation underlying hierarchical and simultaneous utilization of carbon substrates by flux sensors in Escherichia coli.

Many microorganisms exhibit nutrient preferences, exemplified by the 'hierarchical' consumption of certain carbon substrates. Here, we systematically investigate under which physiological conditions hierarchical substrate utilization occurs and its mechanisms of implementation. We show utilization hierarchy of Escherichia coli to be ordered by the carbon-uptake flux rather than the identity of the substrates. A detailed study of glycerol uptake finds that it is fully suppressed if the uptake flux of another glycolytic substrate exceeds a threshold, which is set to the influx obtained when grown on glycerol alone. Below this threshold, limited glycerol uptake is 'supplemented' such that the total carbon uptake is maintained at the threshold. This behaviour results from total-flux feedback mediated by cAMP-Crp signalling but also requires inhibition by the regulator fructose 1,6-bisphosphate, which senses the upper-glycolytic flux and ensures that glycerol uptake defers to other glycolytic substrates but not to gluconeogenic ones. A quantitative model reproduces all of the observed utilization patterns, including those of key mutants. The proposed mechanism relies on the differential regulation of uptake enzymes and requires a specific operon organization. This organization is found to be conserved across related species for several uptake systems, suggesting the deployment of similar mechanisms for hierarchical substrate utilization by a spectrum of microorganisms.

Glucose-Mediated Repression of Plant Biomass Utilization in the White-Rot Fungus Dichomitus squalens.

The extent of carbon catabolite repression (CCR) at a global level is unknown in wood-rotting fungi, which are critical to the carbon cycle and are a source of biotechnological enzymes. CCR occurs in the presence of sufficient concentrations of easily metabolizable carbon sources (e.g., glucose) and involves downregulation of the expression of genes encoding enzymes involved in the breakdown of complex carbon sources. We investigated this phenomenon in the white-rot fungus Dichomitus squalens using transcriptomics and exoproteomics. In D. squalens cultures, approximately 7% of genes were repressed in the presence of glucose compared to Avicel or xylan alone. The glucose-repressed genes included the essential components for utilization of plant biomass-carbohydrate-active enzyme (CAZyme) and carbon catabolic genes. The majority of polysaccharide-degrading CAZyme genes were repressed and included activities toward all major carbohydrate polymers present in plant cell walls, while repression of ligninolytic genes also occurred. The transcriptome-level repression of the CAZyme genes observed on the Avicel cultures was strongly supported by exoproteomics. Protease-encoding genes were generally not glucose repressed, indicating their likely dominant role in scavenging for nitrogen rather than carbon. The extent of CCR is surprising, given that D. squalens rarely experiences high free sugar concentrations in its woody environment, and it indicates that biotechnological use of D. squalens for modification of plant biomass would benefit from derepressed or constitutively CAZyme-expressing strains.IMPORTANCE White-rot fungi are critical to the carbon cycle because they can mineralize all wood components using enzymes that also have biotechnological potential. The occurrence of carbon catabolite repression (CCR) in white-rot fungi is poorly understood. Previously, CCR in wood-rotting fungi has only been demonstrated for a small number of genes. We demonstrated widespread glucose-mediated CCR of plant biomass utilization in the white-rot fungus Dichomitus squalens This indicates that the CCR mechanism has been largely retained even though wood-rotting fungi rarely experience commonly considered CCR conditions in their woody environment. The general lack of repression of genes encoding proteases along with the reduction in secreted CAZymes during CCR suggested that the retention of CCR may be connected with the need to conserve nitrogen use during growth on nitrogen-scarce wood. The widespread repression indicates that derepressed strains could be beneficial for enzyme production.

CreA-independent carbon catabolite repression of cellulase genes by trimeric G-protein and protein kinase A in Aspergillus nidulans.

Cellulase production in filamentous fungi is repressed by various carbon sources. In our preliminary survey in Aspergillus nidulans, degree of de-repression differed depending on carbon sources in a mutant of creA, encoding the transcriptional repressor for carbon catabolite repression (CCR). To further understand mechanisms of CCR of cellulase production, we compared the effects of creA deletion with deletion of protein kinase A (pkaA) and G (ganB) genes, which constitute a nutrient sensing and signaling pathway. In plate culture with carboxymethyl cellulose and D-glucose, deletion of pkaA and ganB, but not creA, led to significant de-repression of cellulase production. In submerged culture with cellobiose and D-glucose or 2-deoxyglucose, both creA or pkaA single deletion led to partial de-repression of cellulase genes with the highest level by their double deletion, while ganB deletion caused de-repression comparable to that of the creA/pkaA double deletion. With ball-milled cellulose and D-glucose, partial de-repression was detected by deletion of creA but not of pkaA or ganB. The creA/pkaA or creA/ganB double deletion led to earlier expression than the creA deletion. Furthermore, the effect of each deletion with D-xylose or L-arabinose as the repressing carbon source was significantly different from that with D-glucose, D-fructose, and D-mannose. Consequently, this study revealed that PkaA and GanB participate in CreA-independent CCR and that contribution of CreA, PkaA, and GanB in CCR differs depending on the inducers, repressing carbon sources, and culture conditions (plate or submerged). Further study of CreA-independent mechanisms is needed to fully understand CCR in filamentous fungi.

Influence of the Hfq and Crc global regulators on the control of iron homeostasis in Pseudomonas putida.

Metabolically versatile bacteria use catabolite repression control to select their preferred carbon sources, thus optimizing carbon metabolism. In pseudomonads, this occurs through the combined action of the proteins Hfq and Crc, which form stable tripartite complexes at target mRNAs, inhibiting their translation. The activity of Hfq/Crc is antagonised by small RNAs of the CrcZ family, the amounts of which vary according to carbon availability. The present work examines the role of Pseudomonas putida Hfq protein under conditions of low-level catabolite repression, in which Crc protein would have a minor role since it is sequestered by CrcZ/CrcY. The results suggest that, under these conditions, Hfq remains operative and plays an important role in iron homeostasis. In this scenario, Crc appears to participate indirectly by helping CrcZ/CrcY to control the amount of free Hfq in the cell. Iron homeostasis in pseudomonads relies on regulatory elements such as the Fur protein, the PrrF1-F2 sRNAs, and several extracytoplasmic sigma factors. Our results show that the absence of Hfq is paralleled by a reduction in PrrF1-F2 small RNAs. Hfq thus provides a regulatory link between iron and carbon metabolism, coordinating the iron supply to meet the needs of the enzymes operational under particular nutritional regimes.

Regulation of metabolism in Escherichia coli during growth on mixtures of the non-glucose sugars: arabinose, lactose, and xylose.

Catabolite repression refers to the process where the metabolism of one sugar represses the genes involved in metabolizing another sugar. While glucose provides the canonical example, many other sugars are also known to induce catabolite repression. However, less is known about the mechanism for catabolite repression by these non-glucose sugars. In this work, we investigated the mechanism of catabolite repression in the bacterium Escherichia coli during growth on lactose, L-arabinose, and D-xylose. The metabolism of these sugars is regulated in a hierarchical manner, where lactose is the preferred sugar, followed by L-arabinose, and then D-xylose. Previously, the preferential utilization of L-arabinose over D-xylose was found to result from transcriptional crosstalk. However, others have proposed that cAMP governs the hierarchical regulation of many non-glucose sugars. We investigated whether lactose-induced repression of L-arabinose and D-xylose gene expression is due to transcriptional crosstalk or cAMP. Our results demonstrate that it is due to cAMP and not transcriptional crosstalk. In addition, we found that repression is reciprocal, where both L-arabinose and D-xylose also repress the lactose gene expression, albeit to a lesser extent and also through a mechanism involving cAMP. Collectively, the results further our understanding of metabolism during growth on multiple sugars.

Concurrent metabolism of pentose and hexose sugars by the polyextremophile Alicyclobacillus acidocaldarius.

Alicyclobacillus acidocaldarius is a thermoacidophilic bacterium capable of growth on sugars from plant biomass. Carbon catabolite repression (CCR) allows bacteria to focus cellular resources on a sugar that provides efficient growth, but also allows sequential, rather than simultaneous use when more than one sugar is present. The A. acidocaldarius genome encodes all components of CCR, but transporters encoded are multifacilitator superfamily and ATP-binding cassette-type transporters, uncommon for CCR. Therefore, global transcriptome analysis of A. acidocaldarius grown on xylose or fructose was performed in chemostats, followed by attempted induction of CCR with glucose or arabinose. Alicyclobacillus acidocaldarius grew while simultaneously metabolizing xylose and glucose, xylose and arabinose, and fructose and glucose, indicating that CCR did not control carbon metabolism. Microarrays showed down-regulation of genes during growth on one sugar compared to two, and occurred primarily in genes encoding: (1) regulators; (2) enzymes for cell wall synthesis; and (3) sugar transporters.

Pectin lyase overproduction by Penicillium griseoroseum mutants resistant to catabolite repression

Abstract Expression of pectinolytic genes is regulated by catabolic repression limiting the production of pectin lyase (PL) if the natural inducer, pectin, is missing from the growth medium. Here, we report the isolation of Penicillium griseoroseum mutants resistant to 2-deoxy-d-glucose (DG) that show resistance to catabolite repression and overproduce PL. Three spontaneous and nine UV-induced mutants were obtained. Some mutants produced sectors (segments morphologically different) that were also studied. The mutants were analyzed for pectinases production on pectinase-agar plates and five mutants and two sectors showing larger clearing zones than the wild type were selected for quantitative assay. Although PL production higher than the wild type has been found, phenotype instability was observed for most of the mutants and, after transfers to nonselective medium, the DG resistance was no longer present. Only mutants M03 and M04 were stable maintaining the DG-resistance phenotype. When growing for 120 h in liquid medium containing glucose with or without pectin, both mutants showed higher PL production. In the presence of glucose as sole carbon source, the mutant M03 produced 7.8-fold more PL than the wild type. Due its phenotypic stability and PL overproduction, the mutant M03 presents potential for industrial applications.

Virulence of Pseudomonas syringae pv. tomato DC3000 Is Influenced by the Catabolite Repression Control Protein Crc.

Pseudomonas syringae infects diverse plant species and is widely used as a model system in the study of effector function and the molecular basis of plant diseases. Although the relationship between bacterial metabolism, nutrient acquisition, and virulence has attracted increasing attention in bacterial pathology, it is largely unexplored in P. syringae. The Crc (catabolite repression control) protein is a putative RNA-binding protein that regulates carbon metabolism as well as a number of other factors in the pseudomonads. Here, we show that deletion of crc increased bacterial swarming motility and biofilm formation. The crc mutant showed reduced growth and symptoms in Arabidopsis and tomato when compared with the wild-type strain. We have evidence that the crc mutant shows delayed hypersensitive response (HR) when infiltrated into Nicotiana benthamiana and tobacco. Interestingly, the crc mutant was more susceptible to hydrogen peroxide, suggesting that, in planta, the mutant may be sensitive to reactive oxygen species generated during pathogen-associated molecular pattern-triggered immunity (PTI). Indeed, HR was further delayed when PTI-induced tissues were challenged with the crc mutant. The crc mutant did not elicit an altered PTI response in plants compared with the wild-type strain. We conclude that Crc plays an important role in growth and survival during infection.

Amino Acid Catabolism in Staphylococcus aureus and the Function of Carbon Catabolite Repression.

Staphylococcus aureus must rapidly adapt to a variety of carbon and nitrogen sources during invasion of a host. Within a staphylococcal abscess, preferred carbon sources such as glucose are limiting, suggesting that S. aureus survives through the catabolism of secondary carbon sources. S. aureus encodes pathways to catabolize multiple amino acids, including those that generate pyruvate, 2-oxoglutarate, and oxaloacetate. To assess amino acid catabolism, S. aureus JE2 and mutants were grown in complete defined medium containing 18 amino acids but lacking glucose (CDM). A mutation in the gudB gene, coding for glutamate dehydrogenase, which generates 2-oxoglutarate from glutamate, significantly reduced growth in CDM, suggesting that glutamate and those amino acids generating glutamate, particularly proline, serve as the major carbon source in this medium. Nuclear magnetic resonance (NMR) studies confirmed this supposition. Furthermore, a mutation in the ackA gene, coding for acetate kinase, also abrogated growth of JE2 in CDM, suggesting that ATP production from pyruvate-producing amino acids is also critical for growth. In addition, although a functional respiratory chain was absolutely required for growth, the oxygen consumption rate and intracellular ATP concentration were significantly lower during growth in CDM than during growth in glucose-containing media. Finally, transcriptional analyses demonstrated that expression levels of genes coding for the enzymes that synthesize glutamate from proline, arginine, and histidine are repressed by CcpA and carbon catabolite repression. These data show that pathways important for glutamate catabolism or ATP generation via Pta/AckA are important for growth in niches where glucose is not abundant, such as abscesses within skin and soft tissue infections.IMPORTANCES. aureus is a significant cause of both morbidity and mortality worldwide. This bacterium causes infections in a wide variety of organ systems, the most common being skin and soft tissue. Within a staphylococcal abscess, levels of glucose, a preferred carbon source, are limited due to the host immune response. Therefore, S. aureus must utilize other available carbon sources such as amino acids or peptides to proliferate. Our results show that glutamate and amino acids that serve as substrates for glutamate synthesis, particularly proline, function as major carbon sources during growth, whereas other amino acids that generate pyruvate are important for ATP synthesis via substrate-level phosphorylation in the Pta-AckA pathway. Our data support a model whereby certain amino acid catabolic pathways, and acquisition of those particular amino acids, are crucial for growth in niches where glucose is not abundant.

Cross-regulation by CrcZ RNA controls anoxic biofilm formation in Pseudomonas aeruginosa.

Pseudomonas aeruginosa (PA) can thrive in anaerobic biofilms in the lungs of cystic fibrosis (CF) patients. Here, we show that CrcZ is the most abundant PA14 RNA bound to the global regulator Hfq in anoxic biofilms grown in cystic fibrosis sputum medium. Hfq was crucial for anoxic biofilm formation. This observation complied with an RNAseq based transcriptome analysis and follow up studies that implicated Hfq in regulation of a central step preceding denitrification. CrcZ is known to act as a decoy that sequesters Hfq during relief of carbon catabolite repression, which in turn alleviates Hfq-mediated translational repression of catabolic genes. We therefore inferred that CrcZ indirectly impacts on biofilm formation by competing for Hfq. This hypothesis was supported by the findings that over-production of CrcZ mirrored the biofilm phenotype of the hfq deletion mutant, and that deletion of the crcZ gene augmented biofilm formation. To our knowledge, this is the first example where competition for Hfq by CrcZ cross-regulates an Hfq-dependent physiological process unrelated to carbon metabolism.

The roles of RelA/(p)ppGpp in glucose-starvation induced adaptive response in the zoonotic Streptococcus suis.

The (p)ppGpp signal molecules play a central role in the stringent response (SR) to adapt to nutrient starvation in bacteria, yet the carbohydrate starvation induced adaptive response and the roles of SR in this response is not well characterized, especially in Gram-positives. Here, two (p)ppGpp synthetases RelA and RelQ are identified in Streptococcus suis, an important emerging zoonotic Gram-positive bacterium, while only RelA is functional under glucose starvation. To characterize the roles of RelA/(p)ppGpp in glucose starvation response in S. suis, the growth curves and transcriptional profiles were compared between the mutant strain ΔrelA [a (p)ppGpp(0) strain under glucose starvation] and its parental strain SC-19 [(p)ppGpp(+)]. The results showed great difference between SC-19 and ΔrelA on adaptive responses when suffering glucose starvation, and demonstrated that RelA/(p)ppGpp plays important roles in adaptation to glucose starvation. Besides the classic SR including inhibition of growth and related macromolecular synthesis, the extended adaptive response also includes inhibited glycolysis, and carbon catabolite repression (CCR)-mediated carbohydrate-dependent metabolic switches. Collectively, the pheno- and genotypic characterization of the glucose starvation induced adaptive response in S. suis makes a great contribution to understanding better the mechanism of SR.

The Regulatory Networks That Control Clostridium difficile Toxin Synthesis.

The pathogenic clostridia cause many human and animal diseases, which typically arise as a consequence of the production of potent exotoxins. Among the enterotoxic clostridia, Clostridium difficile is the main causative agent of nosocomial intestinal infections in adults with a compromised gut microbiota caused by antibiotic treatment. The symptoms of C. difficile infection are essentially caused by the production of two exotoxins: TcdA and TcdB. Moreover, for severe forms of disease, the spectrum of diseases caused by C. difficile has also been correlated to the levels of toxins that are produced during host infection. This observation strengthened the idea that the regulation of toxin synthesis is an important part of C. difficile pathogenesis. This review summarizes our current knowledge about the regulators and sigma factors that have been reported to control toxin gene expression in response to several environmental signals and stresses, including the availability of certain carbon sources and amino acids, or to signaling molecules, such as the autoinducing peptides of quorum sensing systems. The overlapping regulation of key metabolic pathways and toxin synthesis strongly suggests that toxin production is a complex response that is triggered by bacteria in response to particular states of nutrient availability during infection.

Pseudomonas aeruginosa Lon and ClpXP proteases: roles in linking carbon catabolite repression system with quorum-sensing system.

Quorum sensing (QS) plays critical roles in virulence gene expression and the pathogenesis of Pseudomonas aeruginosa, an important human pathogen. However, the regulatory effects, especially that occur directly upstream of the QS system, remain largely unknown. Here, we review recent advances in the understanding of the key component of carbon catabolite repression (CCR) system and protein quality control (PQC) system in regulating the QS system in P. aeruginosa. We propose that PQC proteases Lon and ClpXP may have an important role in linking CCR with QS, and thus contribute to the integration of nutritional cues into the regulatory network governing the virulence factors expression in P. aeruginosa.