Résumé
Microcins are gene-encoded antibacterial peptides secreted by enterobacteria, which act against Gram-negative bacteria and are strongly involved in microbial competitions within the intestinal tract. We have recently isolated a post-translationally modified form of the 84-residue microcin E492 (mccE92m), which exhibits minimal inhibitory concentrations in the nanomolar range. The MccE492m post-translational modification was characterized by biochemical and spectroscopic methods as a trimer of N-(2,3-dihydroxybenzoyl)-L-serine (DHBS) linked via a C-glycosidic linkage to a beta-D-glucose moiety, itself linked to the Ser-84-carboxyl through an O-glycosidic bond. DHBS is a catechol-type siderophore involved in iron uptake by enterobacteria. MccE492m, which bears a siderophore-like post-translational modification, is the first example of a novel type of antibacterial peptide called siderophore-peptide. The MccE492m and MccE492 activity and mechanism of action were studied.We showed that recognition of MccE492m is mediated by iron-siderophore receptors at the surface of bacteria. Moreover, the unmodified form is also recognized by the same receptors. Thus, we propose that MccE492 and MccE492m parasitize iron-siderophore receptors to enter more efficiently into the bacteria and that improvement of MccE492 antimicrobial activity upon modification results from an increase in the microcin/receptor affinity.