Résumé
Microcins are gene-encoded antibacterial peptides secreted by enterobacteria. They are involved in microbial competition within the intestinal tract. Besides the antibacterial peptide, microcin gene clusters encode modification enzymes, export and self-immunity factors. Microcin E492 (MccE492) is a 84-residue peptide secreted by Klebsiella pneumoniae. It is post-translationally modified by a glycosylated linear trimer of N-(2, 3 dihydroxybenzoyl)-L-serine (DHBS) similar to enterobactin/salmochelins. It was the first natural siderophore-peptide to be isolated The siderophore-type post-translational modification was shown to be required for optimal antibacterial activity, probably because the microcin uses the iron-siderophore receptors for recognition at the outer membrane of Escherichia coli. Since under particular culture conditions, MccE492 was also isolated as a less potent unmodified peptide (u-MccE492), we have investigated the maturation process that provides the peptide with higher antibacterial activity. We first showed that the siderophore enterobactin (a cyclic trimer of DHBS) serves as a substrate for MccE492 post-translational modification. Indeed, we found that high iron concentration or free aromatic acids repress both enterobactin synthesis and the acquisition of the MccE492 post-translational modification. Consistently, addition of exogenous enterobactin restored the modification in an enterobactin-deficient strain unable to express mature MccE492. We then identified the enzymes involved in MccE492 maturation through gene-disruption and functional complementation. We showed that both mceC and mceD genes from MccE492 gene cluster are required for MccE492 maturation. These genes, homologous to iroB and iroD, respectively encode a C-glycosyltranferase and an enterobactin-esterase, which convert enterobactin into salmochelins. A tentative model was proposed for MccE492 maturation. Enterobactin would be C-glycosylated by MceC, further hydrolyzed by MceD and finally transferred onto MccE492 Ser84 carboxyl group by unidentified enzymes likely to include MceI and J. Because mceC, D, I and J homologues are found in gene clusters encoding MccH47, MccI47 and/or MccM, this enzyme machinery is believed to be common to all known siderophore-microcins.