Abstract
BACKGROUND: Hemin is a major source of iron for pathogens in infectious niches. The FecIRA-like surface signaling cascade is a common regulatory system for iron acquisition by pathogens. This system consists of a FecA-like TonB-dependent transporter (TBDT), a FecR-like inner membrane anti-sigma factor, and a FecI-like extracytoplasmic function (ECF) sigma factor. Beyond iron acquisition, FecIRA-like systems have been reported to regulate additional physiological processes. The known hemin acquisition system in Stenotrophomonas maltophilia includes HemA, a TBDT; HemU, an inner membrane transporter; and the TonB1-ExbB1-ExbD1a-ExbD1b complex, a multi-subunit motor that energizes HemA. Fur and HemP are the primary regulators involved in hemin utilization. In this study, we identified a novel FecIRA-like regulatory system, i.e., HemI-HemR-HemA(D). METHODS: The regulatory role of HemI was examined using promoter-xylE transcriptional fusion constructs and real-time PCR. Mutants associated with the hemI-hemR-hemA(D) operon were generated and evaluated for iron utilization, swimming motility, oxidative stress tolerance, and antibiotic susceptibility. RESULTS: The hemI-hemR-hemA(D) operon was repressed by Fur-Fe(2+) under iron-replete conditions. Its expression was partially derepressed under iron depletion and further derepressed in the presence of hemin; however, the operon showed no autoregulation. HemI was essential for hemin acquisition. The overexpression of hemI in the S. maltophilia KJ strain increased the susceptibility to levofloxacin (LVX) and trimethoprim-sulfamethoxazole (SXT). All S. maltophilia isolates examined displayed increased minimum inhibitory concentrations (MICs) for ceftazidime (CAZ) and minocycline (MIN) under the iron-depleted and hemin-available conditions; notably, the changes in the MICs of LVX and SXT were strain-dependent. CONCLUSION: HemI, a novel ECF sigma factor, not only regulates hemin acquisition but also contributes to antibiotic susceptibility under iron-limited and hemin-available conditions.