Differential Expression of the Smb Bacteriocin in Streptococcus mutans Isolates

The two-component lantibiotic Smb is produced byStreptococcus mutans GS5. In the present study, we identified seven strains ofS. mutans containing thesmb gene cluster. These strains could be classified into high- and low-level Smb producers relative to the levels of Smb production by indicator strains in vitro. This classification was dependent upon the transcription levels of the structuralsmbA andsmbB genes. Sequence analysis upstream ofsmbA in the high- and low-level Smb-producing strains revealed differences at nucleotide position −46 relative to thesmbA start codon. Interestingly, the transcription start site was present upstream of the point mutation, indicating that both groups of strains have the same promoter constructs and that the differential expression ofsmbA andsmbB mRNA occurred subsequent to transcription initiation. In addition,smbA ::lacZ fusion expression was higher when it was regulated by the sequences of strains with high-level Smb activity than when it was regulated by the comparable region from strains with low-level Smb activity. Taken together, we conclude that high- or low-level Smb expression is dependent on the presence of a G or a T nucleotide at position −46 relative to thesmbA translational start site inS. mutans Smb producers.