Identification and properties of the genes encoding microcin E492 and its immunity protein

The gene coding for the immunity protein (mceB) and the structural gene of microcin E492 (mcd), a low-molecular-weight channel-forming bacteriocin pro duced by a strain of Klebsiella pneumoniae, have been characterized. The mi crocin gene codes for a precursor protein of either 99 or 103 amino acids. Protein sequencing of the N-terminal region of microcin E492 unequivocally identified this gene as the microcin structural gene and indicated that thi s microcin is synthesized as a precursor protein that is cleaved at either amino acid 15 or 19, at a site resembling the double-glycine motif. The gen e encoding the 95 amino-acid immunity protein (mceB) was identified by clon ing the DNA segment that encodes only this polypeptide into an expression v ector and demonstrating the acquisition of immunity to microcin E492. As ex pected, the immunity protein was found to be associated with the inner memb rane. Analysis of the DNA sequence indicates that these genes belong to the same family as microcin 24, and they do not share structural motifs with a ny other known channel-forming bacteriocin. The organization of the microci n- and immunity protein-encoding genes suggests that they are coordinately expressed.