The Bacillus subtilis catabolite control protein CcpA exerts all its regulatory functions by DNA-binding

Glucose-dependent activation of Bacillus anthracis toxin gene expression and virulence requires the carbon catabolite protein CcpA

Sensing environmental conditions is an essential aspect of bacterial physiology and virulence. In Bacillus anthracis, the causative agent of anthrax, transcription of the two major virulence factors, toxin and capsule, is triggered by bicarbonate, a major compound in the mammalian body. Here it is shown that glucose is an additional signaling molecule recognized by B. anthracis for toxin synthesis. The presence of glucose increased the expression of the protective antigen toxin component-encoding gene (pagA) by stimulating induction of transcription of the AtxA virulence transcription factor. Induction of atxA transcription by glucose required the carbon catabolite protein CcpA via an indirect mechanism. CcpA did not bind specifically to any region of the extended atxA promoter. The virulence of a B. anthracis strain from which the ccpA gene was deleted was significantly attenuated in a mouse model of infection. The data demonstrated that glucose is an important host environment-derived signaling molecule and that CcpA is a molecular link between environmental sensing and B. anthracis pathogenesis.

An acetoin-regulated expression system of Bacillussubtilis

An expression system, which is based on the promoter of the acoABCL operon of Bacillus subtilis was developed and characterized. The acoABCL operon codes for the acetoin dehydrogenase complex, which is the major enzyme system responsible for the catabolism of acetoin in B. subtilis. Besides weak organic acids, the neutral overflow metabolite acetoin is metabolized by the cells in the early stationary phase. Transcription of reporter gene fusions with the acoA promoter of this operon is strongly repressed by glucose but induced by acetoin as soon as the preferred carbon source glucose is exhausted. The co-expression of an additional copy of the regulator gene acoR led to more than twofold higher activity of the acoA promoter. It is demonstrated that the induction of this promoter in growing cells with acetoin is possible with non-phosphotransferase system sugars as carbon and energy source and in a ccpA mutant background. Moreover, it could be shown that the activity of the acoA-directed expression system correlates with the level of acetoin in the medium. During glucose limitation, the utilization of the alternative energy source acetoin keeps the protein synthesis machinery of B. subtilis cells active and thus allows for a long lasting acoA-controlled expression of recombinant genes.

Regulation of the glucosyltransferase (gtfBC) operon by CovR in Streptococcus mutans

Streptococcus mutans is an important etiological agent of dental caries in humans. The extracellular polysaccharides synthesized by cell-associated glucosyltransferases (encoded by gtfBC) from sucrose have been recognized as one of the important virulence factors that promote cell aggregation and adherence to teeth, leading to dental plaque formation. In this study, we have characterized the effect of CovR, a global response regulator, on glucosyltransferase expression. Inactivation of covR in strain UA159 resulted in a marked increase in the GtfB and GtfC proteins, as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. With the use of a transcriptional reporter system of a single chromosomal copy of the PgtfB-gusA and PgtfC-gusA fusions, we confirmed the transcriptional regulation of these promoters by CovR. By in vitro electrophoretic mobility shift assays with purified CovR protein, we showed that CovR regulates these promoters directly. DNase I footprinting analyses suggest that CovR binds to large regions on these promoters near the transcription start sites. Taken together, our results indicate that CovR negatively regulates the expression of the gtfB and gtfC genes by directly binding to the promoter region.

The amrG1 gene is involved in the activation of acetate in Corynebacterium glutamicum

During growth of Corynebacterium glutamicum on acetate as its carbon and energy source, the expression of the pta-ack operon is induced, coding for the acetate-activating enzymes, which are phosphotransacetylase (PTA) and acetate kinase (AK). By transposon rescue, we identified the two genes amrG1 and amrG2 found in the deregulated transposon mutant C. glutamicum G25. The amrG1 gene (NCBI-accession: AF532964) has a size of 732 bp, encoding a polypeptide of 243 amino acids and apparently is partially responsible for the regulation of acetate metabolism in C. glutamicum. We constructed an in-frame deletion mutant and an over-expressing strain of amrG1 in the C. glutamicum ATCC13032 wildtype. The strains were then analyzed with respect to their enzyme activities of PTA and AK during growth on glucose, acetate and glucose or acetate alone as carbon sources. Compared to the parental strain, the amrG1 deletion mutant showed higher specific AK and PTA activities during growth on glucose but showed the same high specific activities of AK and PTA on medium containing acetate plus glucose and on medium containing acetate. In contrast to the gene deletion, overexpression of the amrG1 gene in C. glutamicum 13032 had the adverse regulatory effect. These results indicate that the amrG1 gene encodes a repressor or co-repressor of the pta-ack operon.

RegM is required for optimal fructosyltransferase and glucosyltransferase gene expression inStreptococcus mutans

Glucosyltransferases (Gtfs) and fructosyltransferase (Ftf), and the exopolysaccharides they produce, facilitate bacterial adherence and biofilm formation, and enhance the virulence of Streptococcus mutans. In this study, we used continuous chemostat cultures and reporter gene fusions to study the expression of ftf and gtfBC in response to carbohydrate availability and pH, and to asses the role of a protein similar to catabolite control protein A (CcpA), RegM, in regulation of these genes. Expression of ftf was efficient at pH 7.0 and 6.0, but was repressed at pH 5.0 under glucose-excess conditions. At pH 7.0, ftf expression was 5-fold lower under glucose-limiting conditions than in cells growing with an excess of glucose. Expression of gtfBC was also sensitive, albeit to a lesser extent, to pH and glucose availability. Inactivation of regM resulted in decreases of as much as 10-fold in both ftf and gtfBC expression, depending on growth conditions. These findings reinforce the importance of pH and carbohydrate availability for expression of two primary virulence attributes of S. mutans and reveal a critical role for RegM in regulation of expression of both gtfBC and ftf.

The regulatory link between carbon and nitrogen metabolism in Bacillus subtilis: regulation of the gltAB operon by the catabolite control protein CcpA

Bacillus subtilis assimilates ammonium by the concerted action of glutamine synthetase and glutamate synthase. The expression of the gltAB operon encoding the latter enzyme is impaired in B. subtilis ccpA mutant strains. CcpA is a pleiotropic transcriptional regulator that is the key factor in the regulation of carbon metabolism. However, in addition to their defect in catabolite repression ccpA mutants are unable to grow on minimal media with glucose and ammonium as the single sources of carbon and nitrogen, respectively. In this work, the expression of the gltAB operon was analysed and its role in growth on minimal sugar/ammonium media was studied. Expression of gltAB requires induction by glucose or other glycolytically catabolized carbon sources. In ccpA mutants, gltAB cannot be induced by glucose due to the low activity of the phosphotransferase sugar transport system in these mutants. A mutation that allowed phosphotransferase system activity in a ccpA background simultaneously restored glucose induction of gltAB and growth on glucose/ammonium medium. Moreover, artificial induction of the gltAB operon in the ccpA mutant allowed the mutant strain to grow on minimal medium with glucose and ammonium. It may be concluded that expression of the gltAB operon depends on the accumulation of glycolytic intermediates which cannot occur in the ccpA mutant. The lack of gltAB induction is the bottleneck that prevents growth of the ccpA mutant on glucose/ammonium media. The control of expression of the gltAB operon by CcpA provides a major regulatory link between carbon and amino acid metabolism.

Extracellular lipases from Bacillus subtilis: regulation of gene expression and enzyme activity by amino acid supply and external pH

Bacillus subtilis secretes two lipases LipA and LipB into the culture medium. Both enzyme genes were differentially expressed depending on the growth conditions as determined by activity assays and Western blotting in B. subtilis mutant strains lipA, lipB, and the corresponding lipA/lipB double mutant. In minimal medium, LipA was produced at wild-type level in a lipB mutant, however, no LipB protein was detected in a lipA mutant. Interestingly, LipA was produced and secreted at wild-type level in rich medium, but the enzyme remained enzymatically inactive, presumably being caused by a shift of the growth medium to acid pH. Furthermore, expression of the lipase genes was studied using transcriptional fusions with the lacZ reporter gene. The expression of lipA was repressed by high amino acid concentrations, whereas the lipB gene expression remained unaffected.

Bacillus subtilis during feast and famine: visualization of the overall regulation of protein synthesis during glucose starvation by proteome analysis

Dual channel imaging and warping of two-dimensional (2D) protein gels were used to visualize global changes of the gene expression patterns in growing Bacillus subtilis cells during entry into the stationary phase as triggered by glucose exhaustion. The 2D gels only depict single moments during the cells' growth cycle, but a sequential series of overlays obtained at specific points of the growth curve facilitates visualization of the developmental processes at the proteomics scale. During glucose starvation a substantial reprogramming of the protein synthesis pattern was found, with 150 proteins synthesized de novo and cessation of the synthesis of almost 400 proteins. Proteins induced following glucose starvation belong to two main regulation groups: general stress/starvation responses induced by different stresses or starvation stimuli (sigma(B)-dependent general stress regulon, stringent response, sporulation), and glucose-starvation-specific responses (drop in glycolysis, utilization of alternative carbon sources, gluconeogenesis). Using the dual channel approach, it was not only possible to identify those regulons or stimulons, but also to follow the fate of each single protein by the three-color code: red, newly induced but not yet accumulated; yellow, synthesized and accumulated; and green, still present, but no longer being synthesized. These green proteins, which represent a substantial part of the protein pool in the nongrowing cell, are not accessible by using DNA arrays. The combination of 2D gel electrophoresis and MALDI TOF mass spectrometry with the dual channel imaging technique provides a new and comprehensive view of the physiology of growing or starving bacterial cell populations, here for the case of the glucose-starvation response.

Insufficient expression of the ilv-leu operon encoding enzymes of branched-chain amino acid biosynthesis limits growth of a Bacillus subtilis ccpA mutant

Bacillus subtilis ccpA mutant strains exhibit two distinct phenotypes: they are defective in catabolite repression, and their growth on minimal media is strongly impaired. This growth defect is largely due to a lack of expression of the gltAB operon. However, growth is impaired even in the presence of glutamate. Here, we demonstrate that the ccpA mutant strain needs methionine and the branched-chain amino acids for optimal growth. The control of expression of the ilv-leu operon by CcpA provides a novel regulatory link between carbon and amino acid metabolism.