Biosynthesis of gelatinase, a virulence factor of Enterococcus faecalis, was found to be regulated in a cell density-dependent fashion in which its production is active in late log to early stationary phase. Addition of early stationary phase culture filtrate to medium shifted the onset of gelatinase production to that of mid-log phase, suggesting that E. faecalis secretes a gelatinase biosynthesis-activating pheromone (GBAP). GBAP was isolated from culture supernatant of E. faecalis OG1S-P. Structural analysis suggested GBAP to be an 11-residue cyclic peptide containing a lactone structure, in which the -carboxyl group of the C-terminal amino acid is linked to a hydroxyl group of the serine of the third residue. A synthetic peptide possessing the deduced structure showed GBAP activity at nanomolar concentrations as did natural GBAP. Database searches revealed that GBAP corresponds to a C-terminal part of a 242-residue FsrB protein. Northern analysis showed that GBAP slowly induces the transcription of two operons, fsrB-fsrC encoding FsrB and a putative histidine kinase FsrC and gelE-sprE encoding gelatinase GelE and serine protease SprE. Strains with an insertion mutation in either fsrC or a putative response regulator gene fsrA failed to respond to GBAP, suggesting that the GBAP signal is transduced by a two-component regulatory system.