PHYLOGENETIC DISTRIBUTION OF STREPTOCOCCAL SUPERANTIGEN SSA ALLELIC VARIANTS PROVIDES EVIDENCE FOR HORIZONTAL TRANSFER OF SSA WITHIN STREPTOCOCCUS-PYOGENES

Phylogenetic analyses recently found the gene encoding the streptococc al superantigen SSA of Streptococcus pyogenes to occur in several well -differentiated clones comprising 10 (12.5%) of 80 clonal lineages exa mined. To determine if distinct clonal lineages carried the same ssa c oding sequence or harbored a group of allelic variants, ssa was sequen ced 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 s ingle 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 cl onal lineage. It was identical to ssa-3 at codon 94 but had a nonsynon ymous substitution at codon 28 that changed the second amino acid of t he mature protein from serine to arginine. This substitution altered t he predicted isoelectric point and affected the apparent molecular mas s during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. De spite sequence variation both upstream of and within the ssa locus, al l ssa-positive lineages expressed either SSA-1 or SSA-2. The observed patterns of ssa allele-clone distribution provide evidence for individ ual incidences of horizontal transfer and recombination of ssa among d istinct group A streptococcal lineages. Although the extensive homolog y of SSA to the staphylococcal superantigen SEB raises the possibility of intergeneric gene transfer, a search for ssa in 68 genetically div erse clones of Staphylococcus aureus did not identify the gene. Moreov er, the absence of ssa among 119 representative strains of Lancefield group B, C, or G streptococci suggests that ssa is confined to S. pyog enes.