Structure, organization and characterization of the gene cluster involved in the production of microcin E492, a channel-forming bacteriocin

Microcin E492 is a low-molecular-weight, channel-forming bacteriocin produc ed and excreted by Klebsiella pneumoniae RYC492. A 13 kb chromosomal DNA fr agment from K. pneumoniae RYC492 was sequenced, and it was demonstrated by random Tn5 mutagenesis that most of this segment, which has at least 10 cis trons, is needed for the production of active microcin and its immunity pro tein. Genes mceG and mceH correspond to an ABC exporter and its accessory p rotein, respectively, and they are closely related to the colicin V ABC exp ort system. The microcin E492 system also requires the product of gene mceF as an additional factor for export. Despite the fact that this bacteriocin lacks post-translational modifications, genes mceC, mcel and mceJ are need ed for the production of active microcin. Genes mceC and mcel are homologou s to a glycosyl transferase and acyltransferase, respectively, whereas mceJ has no known homologue. Mutants in these three genes secrete an inactive f orm of microcin, able to form ion channels in a phospholipidic bilayer, ind icating that the mutation of these microcin genes does not alter the proces s of membrane insertion. On the other hand, microcin isolated from mutants in genes mceC and mceJ has a lethal effect when incubated with spheroplasts of sensitive cells, indicating that the microcin defects in these mutants are likely to alter receptor recognition at the outer membrane. A model for synthesis and export is proposed as well as a novel maturation pathway tha t would involve conformational changes to explain the production of active microcin E492.