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Infection and Immunity, September 1998, p. 4254-4262, Vol. 66, No. 9
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Cloning and Characterization of tdhA, a Locus Encoding a TonB-Dependent Heme Receptor from Haemophilus ducreyi

Christopher E. Thomas,1 Bonnie Olsen,1 and Christopher Elkins1,2,*

Departments of Medicine1 and Microbiology and Immunology,2 School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599

Received 6 April 1998/Returned for modification 5 May 1998/Accepted 24 June 1998

Haemophilus ducreyi is unable to synthesize heme and must acquire it from its only known host, humans. We cloned and sequenced a gene encoding an outer membrane receptor for heme. It was designated tdhA (for TonB-dependent heme receptor A) since it was related by sequence homology to the family of TonB-dependent receptors. TdhA was strikingly similar to open reading frame HI0113 from the genome of Haemophilus influenzae Rd and also shared homology with five other heme receptors, including HxuC, HemR, HmuR, ChuA, and ShuA, from gram-negative bacteria. An Escherichia coli hemA tonB mutant strongly expressing H. ducreyi tdhA grew on low levels of heme as a source of heme only when an intact H. ducreyi Ton system plasmid was present, formally demonstrating functional TonB dependence. tdhA was expressed poorly in vitro by H. ducreyi and only under conditions of heme limitation. A survey of H. ducreyi revealed that all tested strains but one synthesized small amounts of TdhA in vitro under heme-limiting conditions. Surprisingly, an isogenic mutant of tdhA as well as its parent, 35000, both required the same high levels of heme for growth (50 µg/ml [77 µM] on agar medium). This result, together with previous findings, suggests that in vitro, the uptake of heme by H. ducreyi is mediated by a TonB- and TdhA-independent mechanism, possibly diffusion.

* Corresponding author. Mailing address: Department of Medicine, CB 7030, School of Medicine, University of North Carolina, Chapel Hill, NC 27599. Phone: (919) 966-3661. Fax: (919) 966-6714. E-mail: chriselk{at}med.unc.edu.

Infection and Immunity, September 1998, p. 4254-4262, Vol. 66, No. 9
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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