title: Regulated proteolysis of the alternative sigma factor SigX in Streptococcus mutans: implication in the escape from competence.
abstract: BACKGROUND
  SigX (σX), the alternative sigma factor of Streptococcus mutans, is the key regulator for transcriptional activation of late competence genes essential for taking up exogenous DNA. Recent studies reveal that adaptor protein MecA and the protease ClpC act as negative regulators of competence by a mechanism that involves MecA-mediated proteolysis of SigX by the ClpC in S. mutans. However, the molecular detail how MecA and ClpC negatively regulate competence in this species remains to be determined. Here, we provide evidence that adaptor protein MecA targets SigX for degradation by the protease complex ClpC/ClpP when S. mutans is grown in a complex medium.
  
  
  RESULTS
  By analyzing the cellular levels of SigX, we demonstrate that the synthesis of SigX is transiently induced by competence-stimulating peptide (CSP), but the SigX is rapidly degraded during the escape from competence. A deletion of MecA, ClpC or ClpP results in the cellular accumulation of SigX and a prolonged competence state, while an overexpression of MecA enhances proteolysis of SigX and accelerates the escape from competence. In vitro protein-protein interaction assays confirm that MecA interacts with SigX via its N-terminal domain (NTD1-82) and with ClpC via its C-terminal domain (CTD123-240). Such an interaction mediates the formation of a ternary SigX-MecA-ClpC complex, triggering the ATP-dependent degradation of SigX in the presence of ClpP. A deletion of the N-terminal or C-terminal domain of MecA abolishes its binding to SigX or ClpC. We have also found that MecA-regulated proteolysis of SigX appears to be ineffective when S. mutans is grown in a chemically defined medium (CDM), suggesting the possibility that an unknown mechanism may be involved in negative regulation of MecA-mediated proteolysis of SigX under this condition.
  
  
  CONCLUSIONS
  Adaptor protein MecA in S. mutans plays a crucial role in recognizing and targeting SigX for degradation by the protease ClpC/ClpP. Thus, MecA actually acts as an anti-sigma factor to regulate the stability of SigX during competence development.
authors.forename: Gaofeng
authors.initials: G
authors.lastname: Dong
authors.forename: Xiao-Lin
authors.initials: XL
authors.lastname: Tian
authors.forename: Zubelda A
authors.initials: ZA
authors.lastname: Gomez
authors.affiliation: Department of Applied Oral Sciences, Faculty of Dentistry, Dalhousie University, 5981 University Avenue, Halifax, Nova Scotia B3H 1 W2, Canada. yung-hua.li@dal.ca.
authors.forename: Yung-Hua
authors.initials: YH
authors.lastname: Li
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D003470:Culture Media / Q000737:chemistry; D061269:DNA Transformation Competence; D015964:Gene Expression Regulation, Bacterial; D006360:Heat-Shock Proteins / Q000378:metabolism*; D059748:Proteolysis; D012808:Sigma Factor / Q000378:metabolism*; D013295:Streptococcus mutans / Q000235:genetics*
chemical_list: D001426:Bacterial Proteins; C087481:ClpC protein, Bacteria; D003470:Culture Media; D006360:Heat-Shock Proteins; D012808:Sigma Factor