title: Molecular characterization and phylogenetic distribution of the streptococcal superantigen gene (ssa) from Streptococcus pyogenes.
abstract: A striking increase in the frequency and severity of invasive infections caused by Streptococcus pyogenes has occurred in recent years. Among these diseases is streptococcal toxic-shock-like syndrome (TSLS), a condition characterized by fulminant soft-tissue destruction and multiorgan failure. Streptococcal superantigen (SSA), a superantigen isolated from a TSLS-inducing, serotype M3 S. pyogenes strain, has recently been identified. We here describe the cloning, sequencing, and phylogenetic distribution of the SSA structural gene. The 783-bp open reading frame encodes a predicted 260-amino-acid protein that is similar in size to several other bacterial superantigens. The deduced sequence of the mature protein is 60.2% identical to that of staphylococcal enterotoxin B but only 49% identical to that of streptococcal pyrogenic exotoxin A. Southern blot and PCR analysis of 138 group A streptococcal strains representing 65 M protein serotypes and 15 nontypeable isolates identified ssa in 68 strains from 10 distinct clonal lineages. All ssa-positive clones expressed SSA. Of the two clones associated with TSLS, the ET 2-M3 lineage, but not the ET 1-M1 lineage, carried the SSA gene. Further analysis of the ET 2-M3 lineage found evidence for temporal variation in ssa association. Contemporary ET 2-M3 disease isolates had ssa, but two older isolates of this clone recovered in 1910 and 1920 lacked the gene. The clonal and temporal distribution patterns of ssa suggest a relatively recent acquisition of this superantigen-encoding gene by the ET 2-M3 lineage, perhaps by horizontal transfer and recombination.
authors.affiliation: Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77030.
authors.forename: K B
authors.initials: KB
authors.lastname: Reda
authors.forename: V
authors.initials: V
authors.lastname: Kapur
authors.forename: J A
authors.initials: JA
authors.lastname: Mollick
authors.forename: J G
authors.initials: JG
authors.lastname: Lamphear
authors.forename: J M
authors.initials: JM
authors.lastname: Musser
authors.forename: R R
authors.initials: RR
authors.lastname: Rich
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D008969:Molecular Sequence Data; D010802:Phylogeny; D011994:Recombinant Proteins / Q000096:biosynthesis; D013297:Streptococcus pyogenes / Q000276:immunology*; D013329:Structure-Activity Relationship; D018089:Superantigens / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics*
chemical_list: D011994:Recombinant Proteins; D018089:Superantigens