Hemophore-Dependent Heme Acquisition Systems
Résumé
This chapter describes hemophore secretion and its interaction with heme and hemoproteins, specific receptors, mechanisms of heme transfer from hemophore to receptor, TonB function at various steps in the heme acquisition process, and the regulation of the has operon. HasA hemophores are secreted by ABC transporters. One of the four HasA hemophores identified, the Serratia marcescens HasA protein, has been biochemically characterized. The interaction between HasA and its receptor was first investigated by binding experiments with cells expressing HasR immobilized on nitrocellulose membranes. Second, the binding of HasA to whole cells expressing HasR in liquid media was determined. In a wider perspective, it would be interesting to investigate whether hemophore‐like proteins exist in gram‐positive bacteria and whether hemophores are species specific or are used by nonproducing bacteria, similarly to certain siderophores. Finally, many bacterial species have multiple heme acquisition systems, with only one of these being hemophore dependent. Hemophores enhance the efficiency of heme delivery to bacterial cells and enlarge the heme source range.