Microcin C7 self-immunity protein determines resistance to exogenous microcin C7.
Microcin C7 self-immunity protein (mccF): MccF, a homolog of the LD-carboxypeptidase family, mediates resistance against exogenously added microcin C7 (MccC7), a ribosomally-encoded peptide antibiotic that contains a phosphoramidate linkage to adenosine monophosphate at its C-terminus. The plasmid-encoded mccF gene is transcribed in the opposite direction to the other five genes (mccA-E) and is required for the full expression of immunity but not for production. The catalytic triad residues (Ser, His, Glu) of LD-carboxypeptidase are also conserved in MccF, strongly suggesting that MccF shares the hydrolytic activity with LD-carboxypeptidases. Substrates of MccF have not been deduced, but could likely be microcin C7 precursors. The possible role of MccF is to defend producer cells against exogenous microcin from re-entering after having been exported. It is suggested that MccF is involved in microcin degradation or sequestration in the periplasm.