ThefsrQuorum-Sensing System and Cognate Gelatinase Orchestrate the Expression and Processing of Proprotein EF_1097 into the Mature Antimicrobial Peptide Enterocin O16

A novel antimicrobial peptide designated enterocin O16 was purified fromEnterococcus faecalis. Mass spectrometry showed a monoisotopic mass of 7,231 Da, and N-terminal Edman degradation identified a 29-amino-acid sequence corresponding to residues 90 to 119 of the EF_1097 protein. Bioinformatic analysis showed that enterocin O16 is composed of the 68 most C-terminal residues of the EF_1097 protein. Introduction of an in-frame isogenic deletion in theef1097gene abolished the production of enterocin O16. Enterocin O16 has a narrow inhibitory spectrum, as it inhibits mostly lactobacilli. Apparently,E. faecalisis intrinsically resistant to the antimicrobial peptide, as no immunity connected to the production of enterocin O16 could be identified.ef1097has previously been identified as one of three loci regulated by thefsrquorum-sensing system. The introduction of a nonsense mutation intofsrBconsistently impaired enterocin O16 production, but externally added gelatinase biosynthesis-activating pheromone restored the antimicrobial activity. Functional genetic analysis showed that the EF_1097 proprotein is processed extracellularly into enterocin O16 by the metalloprotease GelE. Thus, it is evident that thefsrquorum-sensing system constitutes the regulatory unit that controls the expression of the EF_1097 precursor protein and the protease GelE and that the latter is required for the formation of enterocin O16. On the basis of these results, this study identified antibacterial antagonism as a novel aspect related to the function offsrand provides a rationale for whyef1097is part of thefsrregulon.IMPORTANCEThefsrquorum-sensing system modulates important physiological functions inE. faecalisvia the activity of GelE. The present study presents a new facet offsrsignaling. The system controls the expression of three primary target operons (fsrABCD,gelE-sprE, andef1097-ef1097b). We demonstrate that the concerted expression of these operons constitutes the elements necessary for the production of a bacteriocin-type peptide and that antimicrobial antagonism is an intrinsic function offsr. The bacteriocin enterocin O16 consists of the 68 most C-terminal residues of the EF_1097 secreted proprotein. The GelE protease processes the EF_1097 proprotein into enterocin O16. In this manner,fsrsignaling enablesE. faecalispopulations to express antimicrobial activity in a cell density-dependent manner.