A new small regulatory protein, HmuP, modulates haemin acquisition in Sinorhizobium meliloti

Sinorhizobium meliloti has multiple systems for iron acquisition, including the use of haem as an iron source. Haem internalization involves the ShmR haem outer membrane receptor and the hmuTUV locus, which participates in haem transport across the cytoplasmic membrane. Previous studies have demonstrated that expression of the shmR gene is negatively regulated by iron through RirA. Here, we identify hmuP in a genetic screen for mutants that displayed aberrant control of shmR. The normal induction of shmR in response to iron limitation was lost in the hmuP mutant, showing that this gene positively affects shmR expression. Moreover, the HmuP protein is not part of the haemin transporter system. Analysis of gene expression and siderophore production indicates that disruption of hmuP does not affect other genes related to the iron-restriction response. Our results strongly indicate that the main function of HmuP is the transcriptional regulation of shmR. Sequence alignment of HmuP homologues and comparison with the NMR structure of Rhodopseudomonas palustris CGA009 HmuP protein revealed that certain amino acids localized within predicted beta-sheets are well conserved. Our data indicate that at least one of the beta-sheets is important for HmuP activity.