Figure 4. The Fsr QS system of E. faecalis.
FsrD, the gelatinase biosynthesis activating pheromone (GBAP) precursor, is processed to a cyclical peptide during secretion by FsrB. Mature GBAP pheromone interacts with the FsrC sensor kinase on the surface of surrounding cells causing phosphorylation of the DNA-binding response regulator, FsrA. Phosphorylated FsrA binds to promoters, including those of fsrB and gelE/sprE, and upregulates gene expression.