Regulatory role of the MisR/S two-component system in hemoglobin utilization in Neisseria meningitidis

Outer membrane iron receptors are some of the major surface entities that are critical for meningococcal pathogenesis. The gene encoding the meningococcal hemoglobin receptor, HmbR, is both independently transcribed and transcriptionally linked to the upstream gene hemO, which encodes a heme oxygenase. The MisR/S two-component system was previously determined to regulate hmbR transcription, and its hemO and hmbR regulatory mechanisms were characterized further here. The expression of hemO and hmbR was downregulated in misR/S mutants under both iron-replete and iron-restricted conditions, and the downregulation could be reversed by complementation. No significant changes in expression of other iron receptors were detected, suggesting that the MisR/S system specifically regulates hmbR. When hemoglobin was the sole iron source, growth defects were detected in the mutants. Primer extension analysis identified a promoter upstream of the hemO-associated Correia element (CE) and another promoter at the proximal end of CE, and processed transcripts previously identified for other cotranscribed CEs were also detected, suggesting that there may be posttranscriptional regulation. MisR directly interacts with sequences upstream of the CE and upstream of the hmbR Fur binding site and thus independently regulates hemO and hmbR. Analysis of transcriptional reporters of hemO and hmbR further demonstrated the positive role of the MisR/S system and showed that the transcription of hmbR initiated from hemO was significantly reduced. A comparison of the effects of the misS mutation under iron-replete and iron-depleted conditions suggested that activation by the MisR/S system and iron-mediated repression by Fur act independently. Thus, the expression of hemO and hmbR is coordinately controlled by multiple independent regulatory mechanisms, including the MisR/S two-component system.