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Infection and Immunity, March 2001, p. 1687-1696, Vol. 69, No. 3
Department of Medicine and Department of
Microbiology and Immunology, Emory University School of
Medicine,1 Research Service, VA Medical
Center,2 and Centers for Disease Control
and Prevention,4 Atlanta, and State
University of West Georgia, Carrollton,3 Georgia
Received 6 September 2000/Returned for modification 5 October
2000/Accepted 27 November 2000
The genetic structure and evolution of a novel exchangeable
meningococcal genomic island was defined for the important human pathogen Neisseria meningitidis. In 125 meningococcal
strains tested, one of three unrelated nucleotide sequences, designated exl (exchangeable locus), was found between a gene
required for heme utilization, hemO, and
col, encoding a putative Escherichia coli
collagenase homologue. The 5' boundary of each exl
cassette was the stop codon of hemO, whereas the 3'
boundary was delineated by a 33-bp repeat containing neisserial uptake
sequences located downstream of col. One of the three
alternative exl cassettes contained the meningococcal
hemoglobin receptor gene, hmbR (exl3). In
other meningococcal strains, hmbR was absent from the
genome and was replaced by either a nucleotide sequence containing a novel open reading frame, exl2, or a cassette
containing exl3. The proteins encoded by
exl2 and exl3 had no significant amino acid homology to HmbR but contained six motifs that are also present in
the lipoprotein components of the lactoferrin (LbpB), transferrin (TbpB), and hemoglobin-haptoglobin (HpuA) uptake systems. To determine the evolutionary relationships among meningococci carrying
hmbR, exl2, or exl3,
isolates representing 92 electrophoretic types were examined.
hmbR was found throughout the population structure of
N. meningitidis (genetic distance, >0.425), whereas
exl2 and exl3 were found in clonal groups
at genetic distances of <0.2. The commensal neisserial species were
identified as reservoirs for all of the exl cassettes
found in meningococci. The structure of these cassettes and their
correlation with clonal groups emphasize the extensive gene pool and
frequent horizontal DNA transfer events that contribute to the
evolution and virulence of N. meningitidis.
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.3.1687-1696.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
exl, an Exchangeable Genetic Island
in Neisseria meningitidis
*
Corresponding author. Present address: Department of
Microbiology, Monash University, Wellington Rd., Clayton 3800, Australia. Phone: 03/99054842. Fax: 03/99054811. E-mail:
charlene.kahler{at}monash.edu.au.
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