title: CcpA represses the expression of the divergent cit operons of Enterococcus faecalis through multiple cre sites.
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.
  
  
  CONCLUSIONS
  In conclusion, our study establishes that expression of the cit operons in E. faecalis is controlled by CCR via CcpA-dependent and -independent mechanisms.
authors: 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.
authors: Cristian A
authors: CA
authors: Suárez
authors: Víctor S
authors: VS
authors: Blancato
authors: Sandrine
authors: S
authors: Poncet
authors: Josef
authors: J
authors: Deutscher
authors: Christian
authors: C
authors: Magni
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D057465:Catabolite Repression; D019343:Citric Acid / Q000378:metabolism*; D015536:Down-Regulation; D013293:Enterococcus faecalis / Q000235:genetics* / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D008969:Molecular Sequence Data; D009876:Operon; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D020218:Response Elements
chemical_list: D001426:Bacterial Proteins; D012097:Repressor Proteins; C069286:catabolite control proteins, bacteria; D019343:Citric Acid