The production in vivo of microcin E492 with antibacterial activity depends on salmochelin and EntF

Microcin E492 is a channel-forming bacteriocin that is found in two forms, namely, a posttranslationally modified form obtained by the covalent linkage of salmochelin-like molecules to serine 84 and an unmodified form. The production of modified microcin E492 requires the synthesis of enterochelin, which is subsequently glycosylated by MceC and converted into salmochelin. mceC mutants produced inactive microcin E492, and this phenotype was reversed either by complementation with iroB from Salmonella enterica or by the addition of exogenous salmochelin. Cyclic salmochelin uptake by Escherichia coli occurred mainly through the outer membrane catecholate siderophore receptor Fiu. The production of inactive microcin E492 by mutants in entB and entC was reverted by the addition of the end product of the respective mutated pathway (2,3-dihydroxybenzoic acid and enterochelin/salmochelin, respectively), while mutants in entF did not produce active microcin E492 in the presence of enterochelin or salmochelin. The EntF adenylation domain was the only domain required for this microcin E492 maturation step. Inactivation of the enzymatic activity of this domain by site-directed mutagenesis did not prevent the synthesis of active microcin E492 in the presence of salmochelin, indicating that the adenylation activity is not essential for the function of EntF at this stage of microcin E492 maturation.