Genetic Analysis of a Unique Bacteriocin, Smb, Produced by Streptococcus mutans GS5

Abstract

A dipeptide lantibiotic, named Smb, in Streptococcus mutans GS5 was characterized by molecular genetic approaches. The Smb biosynthesis gene locus is encoded by a 9.5-kb region of chromosomal DNA and consists of seven genes in the order smbM1 , - T , - F , - M2 , - G , - A , - B . This operon is not present in some other strains of S. mutans , including strain UA159. The genes encoding Smb were identified as smbA and smbB . Inactivation of smbM1 , smbA , or smbB attenuated the inhibition of the growth of the indicator strain RP66, confirming an essential role for these genes in Smb expression. Mature Smb likely consists of the 30-amino-acid SmbA together with the 32-amino-acid SmbB. SmbA exhibited similarity with the mature lantibiotic lacticinA2 from Lactococcus lactis , while SmbB was similar to the mersacidin-like peptides from Bacillus halodurans and L. lactis . We also demonstrated that Smb expression is induced by the competence-stimulating peptide (CSP) and that a com box-like sequence is located in the smb promoter region. These results suggest that Smb belongs to the class I bacteriocin family, and its expression is dependent on CSP-induced quorum sensing.