Identification and characterization of glxR, a gene involved in regulation of glyoxylate bypass in Corynebacterium glutamicum.
ABSTRACT: A corynebacterial clone, previously isolated by scoring repression of lacZYA fused to the aceB promoter of Corynebacterium glutamicum, was analyzed further. In the clone, an open reading frame designated glxR, consisting of 681 nucleotides and encoding a 24,957-Da protein, was found. The molecular mass of a native GlxR protein was estimated by gel filtration column chromatography to be 44,000 Da, suggesting that the protein formed dimers. The predicted amino acid sequence contained both cyclic AMP (cAMP)- and DNA-binding motifs and was homologous with the cAMP receptor protein family of proteins. The aceB-repressing activity of the glxR clone was markedly relieved in an Escherichia coli cya mutant, but the activity was restored in growth medium containing cAMP. In glucose medium, the intracellular cAMP concentration of C. glutamicum reached 22 nmol/mg of protein in the early exponential phase and then decreased further; but in acetate medium, the intracellular cAMP concentration was only 5 nmol/mg of protein and remained low throughout the growth phase. The expression of glxR was not affected by the carbon source. Binding of purified GlxR to the promoter region of aceB could be demonstrated only in the presence of cAMP. These data suggest that GlxR may form dimers which bind to the aceB promoter region in the presence of cAMP and repress the glyoxylate bypass genes.
Project description:Corynebacterium glutamicum GlxR is a cyclic AMP (cAMP) receptor protein-type regulator. Although over 200 GlxR-binding sites in the C. glutamicum genome are predicted in silico, studies on the physiological function of GlxR have been hindered by the severe growth defects of a glxR mutant. This study identified the GlxR regulon by chromatin immunoprecipitation in conjunction with microarray (ChIP-chip) analyses. In total, 209 regions were detected as in vivo GlxR-binding sites. In vitro binding assays and promoter-reporter assays demonstrated that GlxR directly activates expression of genes for aerobic respiration, ATP synthesis, and glycolysis and that it is required for expression of genes for cell separation and mechanosensitive channels. GlxR also directly represses a citrate uptake gene in the presence of citrate. Moreover, ChIP-chip analyses showed that GlxR was still able to interact with its target sites in a mutant with a deletion of cyaB, the sole adenylate cyclase gene in the genome, even though binding affinity was markedly decreased. Thus, GlxR is physiologically functional at the relatively low cAMP levels in the cyaB mutant, allowing the cyaB mutant to grow much better than the glxR mutant.
Project description:Corynebacterium glutamicum GlxR is a homolog of the cAMP receptor protein. Although over 200 GlxR binding sites in the C. glutamicum genome are predicted in silico, studies on the GlxR physiological function have been hindered by the severe growth defects of a glxR mutant. This study comprehensively identified the GlxR regulon by chromatin immunoprecipitation in conjunction with microarray (ChIP-chip) analyses. In total, 209 regions were detected as in vivo GlxR binding sites. Moreover, ChIP-chip analyses showed that GlxR was still able to interact with its target sites in a deletion mutant of cyaB, the sole adenylate cyclase gene in the genome, even though binding affinity was markedly decreased. To identify the direct GlxR targets, we immunoprecipitated DNA from a strain expressing a Strep-tag II-tagged GlxR-protein using an anti-Strep-tag II antibody. To investigate effect of depletion of cAMP by deletion of the cyaB gene, which encodes the sole adenylate cyclase in C. glutamicum, on GlxR binding in vivo, we immunoprecipitated DNA from a cyaB deletion strain expressing a Strep-tag II-tagged GlxR-protein using an anti-Strep-tag II antibody. Three or more independent biological replicates were generated in both cases.
Project description:Corynebacterium glutamicum GlxR is a homolog of the cAMP receptor protein. Although over 200 GlxR binding sites in the C. glutamicum genome are predicted in silico, studies on the GlxR physiological function have been hindered by the severe growth defects of a glxR mutant. This study comprehensively identified the GlxR regulon by chromatin immunoprecipitation in conjunction with microarray (ChIP-chip) analyses. In total, 209 regions were detected as in vivo GlxR binding sites. Moreover, ChIP-chip analyses showed that GlxR was still able to interact with its target sites in a deletion mutant of cyaB, the sole adenylate cyclase gene in the genome, even though binding affinity was markedly decreased. Overall design: To identify the direct GlxR targets, we immunoprecipitated DNA from a strain expressing a Strep-tag II-tagged GlxR-protein using an anti-Strep-tag II antibody. To investigate effect of depletion of cAMP by deletion of the cyaB gene, which encodes the sole adenylate cyclase in C. glutamicum, on GlxR binding in vivo, we immunoprecipitated DNA from a cyaB deletion strain expressing a Strep-tag II-tagged GlxR-protein using an anti-Strep-tag II antibody. Three or more independent biological replicates were generated in both cases.
Project description:The central carbon metabolism genes in Corynebacterium glutamicum are under the control of a transcriptional regulatory network composed of several global regulators. It is known that the promoter region of ramA, encoding one of these regulators, interacts with its gene product, RamA, as well as with the two other regulators, GlxR and SugR, in vitro and/or in vivo. Although RamA has been confirmed to repress its own expression, the roles of GlxR and SugR in ramA expression have remained unclear. In this study, we examined the effects of GlxR binding site inactivation on expression of the ramA promoter-lacZ fusion in the genetic background of single and double deletion mutants of sugR and ramA. In the wild-type background, the ramA promoter activity was reduced to undetectable levels by the introduction of mutations into the GlxR binding site but increased by sugR deletion, indicating that GlxR and SugR function as the transcriptional activator and repressor, respectively. The marked repression of ramA promoter activity by the GlxR binding site mutations was largely compensated for by deletions of sugR and/or ramA. Furthermore, ramA promoter activity in the ramA-sugR double mutant was comparable to that in the ramA mutant but was significantly higher than that in the sugR mutant. Taken together, it is likely that the level of ramA expression is dynamically balanced by GlxR-dependent activation and repression by RamA along with SugR in response to perturbation of extracellular and/or intracellular conditions. These findings add multiple regulatory loops to the transcriptional regulatory network model in C. glutamicum.
Project description:The cyclic AMP-dependent transcriptional regulator GlxR from Corynebacterium glutamicum is a member of the super-family of CRP/FNR (cyclic AMP receptor protein/fumarate and nitrate reduction regulator) transcriptional regulators that play central roles in bacterial metabolic regulatory networks. In C. glutamicum, which is widely used for the industrial production of amino acids and serves as a non-pathogenic model organism for members of the Corynebacteriales including Mycobacterium tuberculosis, the GlxR homodimer controls the transcription of a large number of genes involved in carbon metabolism. GlxR therefore represents a key target for understanding the regulation and coordination of C. glutamicum metabolism. Here we investigate cylic AMP and DNA binding of GlxR from C. glutamicum and describe the crystal structures of apo GlxR determined at a resolution of 2.5 Å, and two crystal forms of holo GlxR at resolutions of 2.38 and 1.82 Å, respectively. The detailed structural analysis and comparison of GlxR with CRP reveals that the protein undergoes a distinctive conformational change upon cyclic AMP binding leading to a dimer structure more compatible to DNA-binding. As the two binding sites in the GlxR homodimer are structurally identical dynamic changes upon binding of the first ligand are responsible for the allosteric behavior. The results presented here show how dynamic and structural changes in GlxR lead to optimization of orientation and distance of its two DNA-binding helices for optimal DNA recognition.
Project description:The genes involved in gluconate catabolism (gntP and gntK) in Corynebacterium glutamicum are scattered in the chromosome, and no regulatory genes are apparently associated with them, in contrast with the organization of the gnt operon in Escherichia coli and Bacillus subtilis. In C. glutamicum, gntP and gntK are essential genes when gluconate is the only carbon and energy source. Both genes contain upstream regulatory regions consisting of a typical promoter and a hypothetical cyclic AMP (cAMP) receptor protein (CRP) binding region but lack the expected consensus operator region for binding of the GntR repressor protein. Expression analysis by Northern blotting showed monocistronic transcripts for both genes. The expression of gntP and gntK is not induced by gluconate, and the gnt genes are subject to catabolite repression by sugars, such as glucose, fructose, and sucrose, as was detected by quantitative reverse transcription-PCR (qRT-PCR). Specific analysis of the DNA promoter sequences (PgntK and PgntP) was performed using bifunctional promoter probe vectors containing mel (involved in melanin production) or egfp2 (encoding a green fluorescent protein derivative) as the reporter gene. Using this approach, we obtained results parallel to those from qRT-PCR. An applied example of in vivo gene expression modulation of the divIVA gene in C. glutamicum is shown, corroborating the possible use of the gnt promoters to control gene expression. glxR (which encodes GlxR, the hypothetical CRP protein) was subcloned from the C. glutamicum chromosomal DNA and overexpressed in corynebacteria; we found that the level of gnt expression was slightly decreased compared to that of the control strains. The purified GlxR protein was used in gel shift mobility assays, and a specific interaction of GlxR with sequences present on PgntP and PgntK fragments was detected only in the presence of cAMP.
Project description:In Corynebacterium glutamicum, cyclic adenosine monophosphate (cAMP) serves as an effector of the global transcriptional regulator GlxR. Synthesis of cAMP is catalyzed by the membrane-bound adenylate cyclase CyaB. In this study, we investigated the consequences of decreased intracellular cAMP levels in a ?cyaB mutant. While no growth defect of the ?cyaB strain was observed on glucose, fructose, sucrose, or gluconate alone, the addition of acetate to these growth media resulted in a severe growth inhibition, which could be reversed by plasmid-based cyaB expression or by supplementation of the medium with cAMP. The effect was concentration- and pH-dependent, suggesting a link to the uncoupling activity of acetate. In agreement, the ?cyaB mutant had an increased sensitivity to the protonophore carbonyl cyanide m-chlorophenyl hydrazone (CCCP). The increased uncoupler sensitivity correlated with a lowered membrane potential of acetate-grown ?cyaB cells compared to wild-type cells. A reduced membrane potential affects major cellular processes, such as ATP synthesis by F1F O -ATP synthase and numerous transport processes. The impaired membrane potential of the ?cyaB mutant could be due to a decreased expression of the cytochrome bc 1-aa 3 supercomplex, which is the major contributor of proton-motive force in C. glutamicum. Expression of the supercomplex genes was previously reported to be activated by GlxR-cAMP. A suppressor mutant of the ?cyaB strain with improved growth on acetate was isolated, which carried a single mutation in the genome leading to an Ala131Thr exchange in GlxR. Introduction of this point mutation into the original ?cyaB mutant restored the growth defect on acetate. This supported the importance of GlxR for the phenotype of the ?cyaB mutant and, more generally, of the cAMP-GlxR system for the control of energy metabolism in C. glutamicum.
Project description:In Corynebacterium glutamicum, the acetate-activating enzymes phosphotransacetylase and acetate kinase and the glyoxylate cycle enzymes isocitrate lyase and malate synthase are coordinately up-regulated in the presence of acetate in the growth medium. This regulation is due to transcriptional control of the respective pta-ack operon and the aceA and aceB genes, brought about at least partly by the action of the negative transcriptional regulator RamB. Using cell extracts of C. glutamicum and employing DNA affinity chromatography, mass spectrometry, and peptide mass fingerprinting, we identified a LuxR-type transcriptional regulator, designated RamA, which binds to the pta-ack and aceA/aceB promoter regions. Inactivation of the ramA gene in the genome of C. glutamicum resulted in mutant RG2. This mutant was unable to grow on acetate as the sole carbon and energy source and, in comparison to the wild type of C. glutamicum, showed very low specific activities of phosphotransacetylase, acetate kinase, isocitrate lyase, and malate synthase, irrespective of the presence of acetate in the medium. Comparative transcriptional cat fusion experiments revealed that this deregulation takes place at the level of transcription. By electrophoretic mobility shift analysis, purified His-tagged RamA protein was shown to bind specifically to the pta-ack and the aceA/aceB promoter regions, and deletion and mutation studies revealed in both regions two binding motifs each consisting of tandem A/C/TG4-6T/C or AC4-5A/G/T stretches separated by four or five arbitrary nucleotides. Our data indicate that RamA represents a novel LuxR-type transcriptional activator of genes involved in acetate metabolism of C. glutamicum.
Project description:The adaptation of Corynebacterium glutamicum to acetate as a carbon and energy source involves transcriptional regulation of the pta-ack operon coding for the acetate-activating enzymes phosphotransacetylase and acetate kinase and of the aceA and aceB genes coding for the glyoxylate cycle enzymes isocitrate lyase and malate synthase, respectively. Deletion and mutation analysis of the respective promoter regions led to the identification of highly conserved 13-bp motifs (AA/GAACTTTGCAAA) as cis-regulatory elements for expression of the pta-ack operon and the aceA and aceB genes. By use of DNA affinity chromatography, a 53-kDa protein specifically binding to the promoter/operator region of the pta-ack operon was purified. Mass spectrometry and peptide mass fingerprinting identified the protein as a putative transcriptional regulator (which was designated RamB). Purified His-tagged RamB protein was shown to bind specifically to both the pta-ack and the aceA/aceB promoter/operator regions. Directed deletion of the ramB gene in the genome of C. glutamicum resulted in mutant strain RG1. Whereas the wild type of C. glutamicum showed high-level specific activities of acetate kinase, phosphotransacetylase, isocitrate lyase, and malate synthase when grown on acetate and low-level specific activities when grown on glucose as sole carbon and energy sources, mutant RG1 showed high-level specific activities with all four enzymes irrespective of the substrate. Comparative transcriptional cat fusion experiments revealed that this deregulation takes place at the level of transcription. The results indicate that RamB is a negative transcriptional regulator of genes involved in acetate metabolism of C. glutamicum.
Project description:The cAMP-dependent transcriptional regulator GlxR serves as a central hub in the regulatory network of the actinobacterial model organism Corynebacterium glutamicum and controls expression of ~10% of all genes. The consequences of a lowered cAMP level are mostly unkown. A single gene (cyaB) for a cAMP-synthesizing adenylate cyclase was identified and it had been reported that a cyaB mutant grows like the wild type on glucose, but has a strong growth defect in the presence of acetate. Our work aimed to understand the molecular basis for the acetate sensitivity of a ΔcyaB mutant and improve our knowledge on the physiological function of cAMP. Our results indicate that a diminished expression of the GlxR-activated genes for the cytochrome bc1-aa3 supercomplex in the ΔcyaB mutant causes a reduced capacity to build up membrane potential. This deficiency becomes evident in the presence of acetate or the protonophore CCCP, which increase the proton permeability of the cytoplasmic membrane, and has systemic effects on transport processes including oxidative phosphorylation. The acetate sensitivity of the ΔcyaB mutant could be abolished by a single amino acid exchange in GlxR. Our results demonstrate a key role of cAMP and GlxR in the control of energy metabolism in corynebacteria. Overall design: DNA microarray analyses of the ΔcyaB mutant and the wild type (WT), grown in minimal medium with glucose and acetate (100 mM each) were performed. For this purpose RNA was isolated from exponentially growing cells cultivated in CGXII medium containing glucose and acetate (each 100 mM) as carbon source. Three biological replicates of both strains were performed.