Immunity to the Staphylococcus aureus leaderless four-peptide bacteriocin aureocin A70 is conferred by AurI, an integral membrane protein.

Aureocin A70, which is produced by Staphylococcus aureus A70, is the only four-component bacteriocin described thus far. The genetic determinants responsible for its production are arranged as three transcriptional units encoded by the 7.9-kb plasmid pRJ6. While the transcriptional unit formed by the genes aurABCD encodes the bacteriocin structural peptides, a second divergent gene, aurT, codes for an ABC transporter involved in bacteriocin externalization. The third transcriptional unit is composed of two genes, orfAB, whose functions were hitherto unknown. RT-PCR analysis of orfAB expression revealed that they are arranged as an operon. When orfAB, either with or without the transcriptional terminator found downstream of orfB, was expressed in two different S. aureus strains sensitive to aureocin A70, all strains became immune to this bacteriocin. Cloning of orfB alone, with or without the transcriptional terminator, confirmed orfB participation in immunity, although full immunity was not observed. An increase in immunity was achieved when two copies of orfB were cloned oriented with the exogenous Plac promoter present in the expression vector pT181mcs. orfB (here referred to as aurI) was shown to be responsible for aureocin A70 immunity, but the full immunity phenotype seems to depend on translational coupling involving orfA, which encodes a putative transcriptional regulator, and aurI.