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CcpA represses the expression of the divergent cit operons of Enterococcus faecalis through multiple cre sites.

Suárez CA, Blancato VS, Poncet S, Deutscher J, Magni C - BMC Microbiol. (2011)

Bottom Line: We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the cit operons.Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that cre1 and cre2 contribute to citHO repression, while cre3 is involved in CCR of citCL.In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.

View Article: PubMed Central - HTML - PubMed

Affiliation: Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET), Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, (S2002LRK) Rosario, Argentina.

ABSTRACT

Background: In Enterococcus faecalis the genes encoding the enzymes involved in citrate metabolism are organized in two divergent operons, citHO and oadHDB-citCDEFX-oadA-citMG (citCL locus). Expression of both operons is specifically activated by adding citrate to the medium. This activation is mediated by binding of the GntR-like transcriptional regulator (CitO) to the cis-acting sequences located in the cit intergenic region. Early studies indicated that citrate and glucose could not be co-metabolized suggesting some form of catabolite repression, however the molecular mechanism remained unknown.

Results: In this study, we observed that the citHO promoter is repressed in the presence of sugars transported by the Phosphoenolpyruvate:carbohydrate Phosphotranserase System (PTS sugars). This result strongly suggested that Carbon Catabolic Repression (CCR) impedes the expression of the activator CitO and the subsequent induction of the cit pathway. In fact, we demonstrate that CCR is acting on both promoters. It is partially relieved in a ccpA-deficient E. faecalis strain indicating that a CcpA-independent mechanism is also involved in regulation of the two operons. Furthermore, sequence analysis of the citH/oadH intergenic region revealed the presence of three putative catabolite responsive elements (cre). We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the cit operons. Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that cre1 and cre2 contribute to citHO repression, while cre3 is involved in CCR of citCL.

Conclusion: In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.

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Effect of different glucose concentrations on the expression of cit promoters in a CitO constitutive genetic background. A and B) JHB15 (JHB11/pTCV-PcitHO) and JHB16 strains (JHB11/pTCV-PcitCL) were grown in LBC (circle) or LBC supplemented with different initial concentrations of glucose: 0.25% (square), 0.5% (up-pointing triangle) and 1% (down-pointing triangle). The corresponding open symbols indicate the remaining glucose concentration in the culture medium (right axis). The levels of accumulated β-galactosidase activity were measured at the time points indicated in the figure. Error bars represent the standard deviation of triplicate measurements. C) Western blot analysis was performed in the complemented CitO deficient strain (JHB11), that was cultivated for 6 h in LB medium supplemented with citrate 1% (LBC) or citrate 1% plus glucose 1% (LBCG).
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Figure 3: Effect of different glucose concentrations on the expression of cit promoters in a CitO constitutive genetic background. A and B) JHB15 (JHB11/pTCV-PcitHO) and JHB16 strains (JHB11/pTCV-PcitCL) were grown in LBC (circle) or LBC supplemented with different initial concentrations of glucose: 0.25% (square), 0.5% (up-pointing triangle) and 1% (down-pointing triangle). The corresponding open symbols indicate the remaining glucose concentration in the culture medium (right axis). The levels of accumulated β-galactosidase activity were measured at the time points indicated in the figure. Error bars represent the standard deviation of triplicate measurements. C) Western blot analysis was performed in the complemented CitO deficient strain (JHB11), that was cultivated for 6 h in LB medium supplemented with citrate 1% (LBC) or citrate 1% plus glucose 1% (LBCG).

Mentions: The divergent organization of the cit genes raises the possibility that the CCR observed could be accomplished by repressing the positive regulator of the pathway (CitO) and the citrate uptake (mediated by CitH). To address this question, CitO was expressed in trans autonomously of the PcitHO promoter (strain JHB11) [6]. In that strain we used the pBM02-derived [28] plasmid, pCitO, in which the expression of citO is under the control of the lactococcal Pcit promoter. As described by Marelli et al., 2010 [28], in E. faecalis expression of different genes put under control of the Pcit promoter was constitutive. In the JHB11-derived strains JHB15 and JHB16 (carrying plasmids pTCV-PcitHO and pTCV-PcitCL, respectively) the activity of the promoters was determined. From Figure 3A it can be seen that in the JHB15 strain repression occurred over the complete range of glucose concentrations tested, whereas in the JHB16 strain (Figure 3B) repression was only noticeable at higher initial glucose concentrations (0.5% (up-pointing triangle) and 1% (down-pointing triangle)). Western blot analysis indicated that CitO levels remained constant in strain JHB11 independently of whether it was grown in presence of citrate (1%) or citrate (1%) and glucose (1%) (Figure 3C). The results presented in Figure 3 suggest that repression of PcitCL is directly mediated by CcpA and that repression of PcitHO is stronger than repression of PcitCL since PcitHO but not PcitCL was repressed at 0.25% initial glucose.


CcpA represses the expression of the divergent cit operons of Enterococcus faecalis through multiple cre sites.

Suárez CA, Blancato VS, Poncet S, Deutscher J, Magni C - BMC Microbiol. (2011)

Effect of different glucose concentrations on the expression of cit promoters in a CitO constitutive genetic background. A and B) JHB15 (JHB11/pTCV-PcitHO) and JHB16 strains (JHB11/pTCV-PcitCL) were grown in LBC (circle) or LBC supplemented with different initial concentrations of glucose: 0.25% (square), 0.5% (up-pointing triangle) and 1% (down-pointing triangle). The corresponding open symbols indicate the remaining glucose concentration in the culture medium (right axis). The levels of accumulated β-galactosidase activity were measured at the time points indicated in the figure. Error bars represent the standard deviation of triplicate measurements. C) Western blot analysis was performed in the complemented CitO deficient strain (JHB11), that was cultivated for 6 h in LB medium supplemented with citrate 1% (LBC) or citrate 1% plus glucose 1% (LBCG).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3198936&req=5

Figure 3: Effect of different glucose concentrations on the expression of cit promoters in a CitO constitutive genetic background. A and B) JHB15 (JHB11/pTCV-PcitHO) and JHB16 strains (JHB11/pTCV-PcitCL) were grown in LBC (circle) or LBC supplemented with different initial concentrations of glucose: 0.25% (square), 0.5% (up-pointing triangle) and 1% (down-pointing triangle). The corresponding open symbols indicate the remaining glucose concentration in the culture medium (right axis). The levels of accumulated β-galactosidase activity were measured at the time points indicated in the figure. Error bars represent the standard deviation of triplicate measurements. C) Western blot analysis was performed in the complemented CitO deficient strain (JHB11), that was cultivated for 6 h in LB medium supplemented with citrate 1% (LBC) or citrate 1% plus glucose 1% (LBCG).
Mentions: The divergent organization of the cit genes raises the possibility that the CCR observed could be accomplished by repressing the positive regulator of the pathway (CitO) and the citrate uptake (mediated by CitH). To address this question, CitO was expressed in trans autonomously of the PcitHO promoter (strain JHB11) [6]. In that strain we used the pBM02-derived [28] plasmid, pCitO, in which the expression of citO is under the control of the lactococcal Pcit promoter. As described by Marelli et al., 2010 [28], in E. faecalis expression of different genes put under control of the Pcit promoter was constitutive. In the JHB11-derived strains JHB15 and JHB16 (carrying plasmids pTCV-PcitHO and pTCV-PcitCL, respectively) the activity of the promoters was determined. From Figure 3A it can be seen that in the JHB15 strain repression occurred over the complete range of glucose concentrations tested, whereas in the JHB16 strain (Figure 3B) repression was only noticeable at higher initial glucose concentrations (0.5% (up-pointing triangle) and 1% (down-pointing triangle)). Western blot analysis indicated that CitO levels remained constant in strain JHB11 independently of whether it was grown in presence of citrate (1%) or citrate (1%) and glucose (1%) (Figure 3C). The results presented in Figure 3 suggest that repression of PcitCL is directly mediated by CcpA and that repression of PcitHO is stronger than repression of PcitCL since PcitHO but not PcitCL was repressed at 0.25% initial glucose.

Bottom Line: We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the cit operons.Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that cre1 and cre2 contribute to citHO repression, while cre3 is involved in CCR of citCL.In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.

View Article: PubMed Central - HTML - PubMed

Affiliation: Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET), Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, (S2002LRK) Rosario, Argentina.

ABSTRACT

Background: In Enterococcus faecalis the genes encoding the enzymes involved in citrate metabolism are organized in two divergent operons, citHO and oadHDB-citCDEFX-oadA-citMG (citCL locus). Expression of both operons is specifically activated by adding citrate to the medium. This activation is mediated by binding of the GntR-like transcriptional regulator (CitO) to the cis-acting sequences located in the cit intergenic region. Early studies indicated that citrate and glucose could not be co-metabolized suggesting some form of catabolite repression, however the molecular mechanism remained unknown.

Results: In this study, we observed that the citHO promoter is repressed in the presence of sugars transported by the Phosphoenolpyruvate:carbohydrate Phosphotranserase System (PTS sugars). This result strongly suggested that Carbon Catabolic Repression (CCR) impedes the expression of the activator CitO and the subsequent induction of the cit pathway. In fact, we demonstrate that CCR is acting on both promoters. It is partially relieved in a ccpA-deficient E. faecalis strain indicating that a CcpA-independent mechanism is also involved in regulation of the two operons. Furthermore, sequence analysis of the citH/oadH intergenic region revealed the presence of three putative catabolite responsive elements (cre). We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the cit operons. Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that cre1 and cre2 contribute to citHO repression, while cre3 is involved in CCR of citCL.

Conclusion: In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.

Show MeSH
Related in: MedlinePlus