Differential expression of the Smb bacteriocin in Streptococcus mutans isolates

The two-component lantibiotic Smb is produced by Streptococcus mutans GS5. In the present study, we identified seven strains of S. mutans containing the smb 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 structural smbA and smbB genes. Sequence analysis upstream of smbA in the high- and low-level Smb-producing strains revealed differences at nucleotide position -46 relative to the smbA 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 of smbA and smbB 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 the smbA translational start site in S. mutans Smb producers.