The Small Protein HemP Is a Transcriptional Activator for the Hemin Uptake Operon in Burkholderia multivorans ATCC 17616.

Iron and heme play very important roles in various metabolic functions in bacteria, and their intracellular homeostasis is maintained because high concentrations of free forms of these molecules greatly facilitate the Fenton reaction-mediated production of large amounts of reactive oxygen species that severely damage various biomolecules. The ferric uptake regulator (Fur) from ATCC 17616 is an iron-responsive global transcriptional regulator, and its deletant exhibits pleiotropic phenotypes. In this study, we found that the phenotypes of the deletant were suppressed by an additional mutation in The transcription of was negatively regulated by Fur under iron-replete conditions and was constitutive in the deletant. Growth of a deletant was severely impaired in a medium containing hemin as the sole iron source, demonstrating the important role of HemP in hemin utilization. HemP was required as a transcriptional activator that specifically binds the promoter-containing region upstream of a Fur-repressive operon, which encodes the proteins for hemin uptake. A deletant was still able to grow using hemin as the sole iron source, albeit at a rate clearly lower than that of the wild-type strain. These results strongly suggested (i) the involvement of HmuR in hemin uptake and (ii) the presence in ATCC 17616 of at least part of other unknown hemin uptake systems whose expression depends on the HemP function. Our analysis also indicated high-affinity binding of HemP to hemin, and such a property might modulate transcriptional activation of the operon. Although the genes for the utilization of hemin as a sole iron source have been identified in a few strains, the regulatory expression of these genes has remained unknown. Our analysis in this study using ATCC 17616 showed that its HemP protein is required for expression of the operon, and the role of HemP in betaproteobacterial species was elucidated for the first time, to our knowledge, in this study. The HemP protein was also found to have two additional properties that have not been reported for functional homologues in other species; one is that HemP is able to bind to the promoter-containing region of the operon to directly activate its transcription, and the other is that HemP is also required for the expression of an unknown hemin uptake system.