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Infect. Immun., 04 1996, 1161-1165, Vol 64, No. 4
KB Reda, V Kapur, D Goela, JG Lamphear, JM Musser and R Rich R
Phylogenetic analyses recently found the gene encoding the streptococcal
superantigen SSA of Streptococcus pyogenes to occur in several
well-differentiated clones comprising 10 (12.5%) of 80 clonal lineages
examined. To determine if distinct clonal lineages carried the same ssa
coding sequence or harbored a group of allelic variants, ssa was sequenced
from 23 S. pyogenes strains representing the 10 clones identified by
multilocus enzyme electrophoresis. Three alleles of ssa were found in
natural populations of S. pyogenes. ssa-1 and ssa-3 differed by a single
synonymous substitution in codon 94; both encoded SSA-1. Each of these
alleles was present in phylogenetically diverse clones that had not shared
a recent common ancestor. ssa-2 was present in a single clonal lineage. It
was identical to ssa-3 at codon 94 but had a nonsynonymous substitution at
codon 28 that changed the second amino acid of the mature protein from
serine to arginine. This substitution altered the predicted isoelectric
point and affected the apparent molecular mass during sodium dodecyl
sulfate-polyacrylamide gel electrophoresis. Despite sequence variation both
upstream of and within the ssa locus, all ssa-positive lineages expressed
either SSA-1 or SSA-2. The observed patterns of ssa allele-clone
distribution provide evidence for individual incidences of horizontal
transfer and recombination of ssa among distinct group A streptococcal
lineages. Although the extensive homology of SSA to the staphylococcal
superantigen SEB raises the possibility of intergeneric gene transfer, a
search for ssa in 68 genetically diverse clones of Staphylococcus aureus
did not identify the gene. Moreover, the absence of ssa among 119
representative strains of Lancefield group B, C, or G streptococci suggests
that ssa is confined to S. pyogenes.
Copyright © 1996, American Society for Microbiology
Phylogenetic distribution of streptococcal superantigen SSA allelic variants provides evidence for horizontal transfer of ssa within Streptococcus pyogenes
Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77030, USA.
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