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J. Bacteriol., Feb 1997, 805-812, Vol 179, No. 3
I Stojiljkovic and N Srinivasan
We have recently cloned and characterized the hemoglobin (Hb) receptor
gene, hmbR, from Neisseria meningitidis. To identify additional proteins
that are involved in Hb utilization, the N. meningitidis Hb utilization
system was reconstituted in Escherichia coli. Five cosmids from N.
meningitidis DNA library enabled a heme-requiring (hemA), HmbR- expressing
mutant of E. coli to use Hb as both porphyrin and iron source. Nucleotide
sequence analysis of DNA fragments subcloned from the Hb-complementing
cosmids identified four open reading frames, three of them homologous to
Pseudomonas putida, E. coli, and Haemophilus influenzae exbB, exbD, and
tonB genes. The N. meningitidis TonB protein is 28.8 to 33.6% identical to
other gram-negative TonB proteins, while the N. meningitidis ExbD protein
shares between 23.3 and 34.3% identical amino acids with other ExbD and
TolR proteins. The N. meningitidis ExbB protein was 24.7 to 36.1%
homologous with other gram- negative ExbB and TolQ proteins.
Complementation studies indicated that the neisserial Ton system cannot
interact with the E. coli FhuA TonB- dependent outer membrane receptor. The
N. meningitidis tonB mutant was unable to use Hb, Hb-haptoglobin complexes,
transferrin, and lactoferrin as iron sources. Insertion of an antibiotic
cassette in the 3' end of the exbD gene produced a leaky phenotype.
Efficient usage of heme by N. meningitidis tonB and exbD mutants suggests
the existence of a Ton-independent heme utilization mechanism. E. coli
complementation studies and the analysis of N. meningitidis hmbR and hpu
mutants suggested the existence of another Hb utilization mechanism in this
organism.
Copyright © 1997, American Society for Microbiology
Neisseria meningitidis tonB, exbB, and exbD genes: Ton-dependent utilization of protein-bound iron in Neisseriae
Department of Microbiology and Immunology, Emory University, Atlanta, Georgia 30322, USA. stojiljk@microbio.emory.edu
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