Streptococcus mutans， a causative agent of dental caries， relies on multiple quorum-sensing (QS) pathways that coordinate the expression of factors needed for colonization in the oral cavity. S. mutans uses small peptides as QS signaling molecules that typically are secreted into the outside milieu. Competence-stimulating peptide (CSP) is one such QS signaling molecule that functions through the ComDE two-component signal transduction pathway. CSP is secreted through NlmTE， a dedicated ABC transporter that cleaves off the N-terminal leader peptide to generate a mature peptide that is 21 residues long (CSP-21). We recently identified a surface-localized protease， SepM， which further cleaves the CSP-21 peptide at the C-terminal end and removes the last 3 residues to generate CSP-18. CSP-18 is the active QS molecule that interacts with the ComD sensor kinase to activate the QS pathway. In this study， we show that SepM specifically cleaves CSP-21 between the Ala18 and Leu19 residues. We also show that SepM recognizes only Ala at position 18 and Leu at position 19， although some CSP-18 variants with a substitution at position 18 can function equally as well as the QS peptide. Furthermore， we demonstrate that SepM homologs from other streptococci are capable of processing CSP-21 to generate functional CSP-18.