The regulatory mechanisms that control the ESX-1 secretion system, a key player in the pathogenesis of , have not been fully elucidated. However, factors that regulate the ESX-1 substrate EspA usually affect ESX-1 function. Previous studies showed that is directly regulated by the nucleoid-associated protein EspR and the two-component system (TCS) MprAB. The PhoPR TCS also activates , but the direct target of PhoP was unknown. In this report, we reveal that EspR is directly regulated by MprA and PhoP-Rv, but not by PhoP-Ra. PhoP-Rv and MprA binding sites in the promoter were determined by gel-shift and DNase I footprinting assays, which identified a PhoP-protected region centred approximately 205 bp before the start codon and that encompasses MprA Region-1, one of two MprA-protected regions. MprA Region-2 is located approximately 60 bp downstream of MprA Region-1 and overlaps a known EspR binding site. Nucleotides essential for the binding of PhoP and/or MprA were identified through site-directed DNA mutagenesis. Our studies also indicate that MprA Region-2, but not MprA Region-1/PhoP region, is required for the full expression of . Recombinant strains carrying mutations at MprA Region-2 exhibited lower transcription levels for , and , and had reduced EspR and EspA levels in cell lysates. These findings indicate that EspR may mediate the regulatory effect of PhoPR and MprAB, and provide more insight into the mechanisms underlying ESX-1 control.


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