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Infection and Immunity, March 2001, p. 1373-1380, Vol. 69, No. 3
Veterans Affairs Medical Center and
University of California San Francisco, San Francisco,
California,1 and Children's
Hospital, Seattle, Washington2
Received 1 September 2000/Returned for modification 3 November
2000/Accepted 27 November 2000
The direct binding of bacteria to platelets is a postulated major
interaction in the pathogenesis of infective endocarditis. To identify
bacterial components that mediate platelet binding by
Streptococcus mitis, we screened a
Tn916
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.3.1373-1380.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Genetic Loci of Streptococcus mitis That
Mediate Binding to Human Platelets
E-derived mutant library of S. mitis
strain SF100 for reduced binding to human platelets in vitro. Two
distinct loci were found to affect platelet binding. The first contains
a gene (pblT) encoding a highly hydrophobic, 43-kDa protein
with 12 potential membrane-spanning segments. This protein resembles
members of the major facilitator superfamily of small-molecule
transporters. The second platelet binding locus consists of an apparent
polycistronic operon. This region includes genes that are highly
similar to those of Lactococcus lactis phage r1t and
Streptococcus thermophilus phage 01205. Two genes
(pblA and pblB) encoding large surface proteins
are also present. The former encodes a 107-kDa protein containing
tryptophan-rich repeats, which may serve to anchor the protein within
the cell wall. The latter encodes a 121-kDa protein most similar to a
tail fiber protein from phage 01205. Functional mapping by
insertion-duplication mutagenesis and gene complementation indicates
that PblB may be a platelet adhesin and that expression of PblB may be
linked to that of PblA. The combined data indicate that at least two
genomic regions contribute to platelet binding by S. mitis.
One encodes a probable transmembrane transporter, while the second
encodes two large surface proteins resembling structural components of lysogenic phages.
*
Corresponding author. Mailing address: Division of
Infectious Diseases, VA Medical Center (111W), 4150 Clement St., San
Francisco, CA 94121. Phone: (415) 221-4810, ext. 2550. Fax: (415)
750-0502. E-mail: sullam{at}itsa.ucsf.edu.
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