Decoding the Microcin J25 Biosynthetic Cluster: Modulation of the mcjA Promoter by the Novel Overlapping Gene mcjX

A comprehensive analysis of the microcin J25 (MccJ25) biosynthetic gene cluster revealed a previously uncharacterized 96-base pair overlapping gene, designated mcjX. This gene features a +1 reading frame shift relative to the primary sequence and encodes a 31-amino acid peptide. Notably, 53 nucleotides overlap with the 3′ terminus of the structural gene mcjA. Such significant overlaps are rare features in the Escherichia coli genome, highlighting the hidden complexity of microbial operon architectures. In this study, we demonstrate that mcjX is actively translated. Functional assays, including green fluorescent protein reporter systems, suggest that McjX acts as a negative regulator of the mcjA promoter, modulating MccJ25 expression. This discovery represents the first report of a regulatory mechanism mediated by an overlapping gene within a lasso peptide operon, providing new perspectives on how microbial genomes fine-tune the production of antimicrobial peptides through compact genetic organization.