Objectives: PrgX, a key player in controlling conjugation induced by the peptide pheromone cCF10 in Enterococcus faecalis, is the cytoplasmic receptor for the cCF10 peptide pheromone and has been shown to bind to two sequences in the intergenic region of pCF10 between prgX and prgQ (the prgQ operon encodes the conjugative transfer functions of pCF10). iCF10 is a plasmid-encoded peptide competitor of cCF10. Elucidating the structures of PrgX will reveal the mechanism of PrgX regulating the cell-cell conjugation gene expression. Methods: We have solved both crystal structures of PrgX/cCF10 and PrgX/iCF10 complexes after solving the PrgX crystal structure. Results: PrgX contains an N-terminal DNA-binding domain, a large central dimerization domain and a C-terminal regulatory domain. The structure of the PrgX/cCF10 complex reveals that pheromone binds in the cleft of the central dimerization domain, causes the C-terminal regulatory domain rotates about 120º. Amino acids 304-317 are mobile and invisible in either the PrgX or PrgX/cCF10 structures. The iCF10 binds in the pheromone-binding pocket the same way as cCF10 and the C-terminal domain in PrgX/iCF10 complex keeps the same conformation as in uncomplexed PrgX. Conclusions: We proposed that PrgX functions as a tetramer in vivo and the binding of CF10 disrupts the PrgX tetramer. iCF10 stabilizes the "native conformation" of PrgX and therefore the PrgX tetramer and thus enhancing PrgX repression of the prgQ operon. cCF10 and iCF10 have very similar chemical composites (both are heptapeptide), but have complete different biological functions. The biological functional difference between cCF10 and iCF10 is determined by multiple amino acids.

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