Contrasting regulation of the ecp common pilus operon in Citrobacter rodentium: essential roles of a distal element and IHF even in the absence of H-NS.

The Escherichia coli common pilus (ECP) is a chaperone-usher pilus widely distributed in E. coli and other enterobacteria, where it mediates adhesion, biofilm formation, and colonization. Citrobacter rodentium, a surrogate model for the human attaching-and-effacing pathogens enteropathogenic and enterohemorrhagic E. coli, contains a conserved ecp operon (Cr-ecp) whose regulation has substantially diverged from E. coli. Cr-ecp expression is induced under static DMEM growth at temperatures below 37 °C, driven by a σ⁷⁰-dependent promoter embedded in a regulatory region of approximately 410 bp. We identified an essential distal regulatory element (DRE) between positions - 378 and - 359 relative to the transcription start site, with a 10-bp core required for promoter activity. In contrast to the EcpR-dependent model in E. coli, the ecpR ortholog is dispensable under the conditions tested. The nucleoid-associated proteins IHF and H-NS bind the regulatory region; IHF is strictly required for activation even when H-NS is absent. These findings define a regulatory architecture distinct from E. coli ecp.