SMU.152 Acts as an Immunity Protein for Mutacin IV

Streptococcus mutans, a principal causative agent of dental caries, secretes antimicrobial peptides known as mutacins to sup-press the growth of competing species to establish a successful colonization. S. mutansUA159, a sequenced strain, produces at least twomajor mutacins, mutacins IV and V. Mutacin IV is a two-peptide mutacin encoded by nlmAB genes, which are mapped just upstream of a putative immunity-encoding gene SMU.152. Here we explored the function of SMU.152 as an immunity pro-tein. We observed that overexpression of SMU.152 in two sensitive host strains converted the strains to become immune to mu-tacin IV. To identify the residues that are important for immunity function, we sequentially deleted residues from the C-termi-nal region of SMU.152. We observed that deletion of as few as seven amino acids, all of which are highly charged (KRRSKNK), drastically reduced the immunity function of the protein. Furthermore, we identified two other putative immunity proteins, SMU.1909 and SMU.925, which lack the last four charged residues (SKNK) that are present in SMU.152 but contain the KRR residues. Synthetic addition of SKNK residues to either SMU.1909 or SMU.925 to reconstitute the KRRSKNKmotif and express-ing these constructs in sensitive cells rendered the cells resistant to mutacin IV.We also demonstrated that deletion of Man-PTS system from a sensitive strain made the cells partially resistant to mutacin IV, indicating that the Man-PTS system plays a role in mutacin IV recognition. Bacteriocins are ribosomally synthesized small antimicrobialpeptides largely produced by lactic acid bacteria and are active against a wide range of bacterial species (8, 27). Bacteriocins ar