The Bordetella bhu locus is required for heme iron utilization

Bordetella pertussis and Bordetella bronchiseptica are capable of obtaining iron from hemin and hemoglobin. Genes encoding a putative bacterial heme i ron acquisition system (bhu, for Bordetella heme utilization) were identifi ed in a B. pertussis genomic sequence database, and the corresponding DNA w as isolated from a virulent strain of B. pertussis. A B. pertussis bhuR mut ant, predicted to lack the heme outer membrane receptor, was generated by a llelic exchange. In contrast to the wild-type strain, bhuR mutant PM5 was i ncapable of acquiring iron from hemin and hemoglobin; genetic complementati on of PM5 with the cloned bhuRSTUV genes restored heme utilization to wild- type levels. In parallel studies, B. bronchiseptica bhu sequences were also identified and a B. bronchiseptica bhuR mutant was constructed and confirm ed to be defective in heme iron acquisition. The wild-type B. bronchiseptic a parent strain grown under low-iron conditions produced the presumptive Bh uR protein, which was absent in the bhuR mutant. Furthermore, production of BhuR by iron-starved B. bronchiseptica was markedly enhanced by culture in hemin-supplemented medium, suggesting that these organisms sense and respo nd to heme in the environment. Analysis of the genetic region upstream of t he bhu cluster identified open reading frames predicted to encode homologs of the Escherichia coli ferric citrate uptake regulators FecI and FecR. The se putative Bordetella regulators may mediate heme-responsive positive tran scriptional control of the bhu genes.