OmpR-Mediated Transcriptional Regulation and Function of Two Heme Receptor Proteins of Bio-Serotype 2/O:9.

We show that strain Ye9 (bio-serotype 2/O:9) utilizes heme-containing molecules as an iron source. The Ye9 genome contains two multigenic clusters, -1 and -2, encoding putative heme receptors HemR1 and HemR2, that share 62% amino acid identity. Expression of these proteins in an mutant defective in heme biosynthesis allowed this strain to use hemin and hemoglobin as a source of porphyrin. The -1 and -2 clusters are organized as operons, expressed from the p and weaker p promoters, respectively. Expression of both operons is negatively regulated by iron and the iron-responsive transcriptional repressor Fur. In addition, OmpR, the response regulator of two component system (TCSs) EnvZ/OmpR, represses transcription of both operons through interaction with binding sequences overlapping the -35 region of their promoters. Western blot analysis of the level of HemR1 in , and mutants, showed an additive effect of these mutations, indicating that OmpR may regulate HemR expression independently of Fur. However, the effect of OmpR on the activity of the p promoter and on HemR1 production was observed in both iron-depleted and iron-replete conditions, i.e., when Fur represses the iron-regulated promoter. In addition, a hairpin RNA thermometer, composed of four uracil residues (FourU) that pair with the ribosome-binding site in the 5'-untranslated region (5'-UTR) of was predicted by analysis. However, thermoregulated expression of HemR1 could not be demonstrated. Taken together, these data suggest that Fur and OmpR control iron/heme acquisition via a complex mechanism based on negative regulation of and at the transcriptional level. This interplay could fine-tune the level of heme receptor proteins to allow to fulfill its iron/heme requirements without over-accumulation, which might be important for pathogenic growth within human hosts.