title: Role of the Haemophilus ducreyi Ton system in internalization of heme from hemoglobin.
abstract: By cloning into Escherichia coli and construction of isogenic mutants of Haemophilus ducreyi, we showed that the hemoglobin receptor (HgbA) is TonB dependent. An E. coli hemA tonB mutant expressing H. ducreyi hgbA grew on low levels of hemoglobin as a source of heme only when an intact H. ducreyi Ton system plasmid was present. In contrast, growth on heme by the E. coli hemA tonB mutant expressing hgbA was observed only at high concentrations of heme, was TonB independent, and demonstrated that H. ducreyi HgbA was not sufficient to function as a typical TonB-dependent heme receptor in E. coli. Allelic replacement of the wild-type H. ducreyi exbB, exbD, and tonB loci with the exbB, exbD, and tonB deletion resulted in an H. ducreyi isogenic mutant unable to utilize hemoglobin but able to utilize hemin at the same levels as the parent strain to fulfill its heme requirement. This finding confirms the TonB dependence of HgbA-mediated hemoglobin utilization and suggests that uptake of hemin in H. ducreyi is TonB independent. Additionally, the H. ducreyi Ton system mutant synthesized increased amounts of HgbA and other heme-regulated outer membrane proteins, consistent with derepression of these proteins due to lower intracellular heme and/or iron concentrations in the mutant. Sequencing of the Ton system genes revealed that the arrangement of the genes was exbB exbD tonB. The proximity and structure of these genes suggested that they are transcribed as an operon. This arrangement, as well as the DNA and deduced amino acid sequences of these H. ducreyi genes, was most similar to those from other pasteurellae.
authors.affiliation: Department of Medicine, School of Medicine, University of North Carolina, Chapel Hill 27599, USA. chriselk@med.unc.edu
authors.forename: C
authors.initials: C
authors.lastname: Elkins
authors.forename: P A
authors.initials: PA
authors.lastname: Totten
authors.forename: B
authors.initials: B
authors.lastname: Olsen
authors.forename: C E
authors.initials: CE
authors.lastname: Thomas
mesh_terms: D000445:Aldehyde Oxidoreductases / Q000235:genetics; D000483:Alleles; D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000032:analysis / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial; D006191:Haemophilus ducreyi / Q000235:genetics* / Q000378:metabolism* / Q000502:physiology; D006418:Heme / Q000378:metabolism*; D006427:Hemin / Q000378:metabolism; D006454:Hemoglobins / Q000378:metabolism*; D007501:Iron / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D008969:Molecular Sequence Data; D005810:Multigene Family; D011995:Recombination, Genetic; D015183:Restriction Mapping; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; C052874:ExbB protein, E coli; D006454:Hemoglobins; D008565:Membrane Proteins; C017206:hemoglobin-binding protein, bacteria; C057616:tonB protein, Bacteria; C489181:tonB protein, E coli; C060790:exbD protein, E coli; D006418:Heme; D006427:Hemin; D007501:Iron; D000445:Aldehyde Oxidoreductases; C067282:glutamyl tRNA reductase