Molecular evolution of a multigene family in group A streptococci

The emm genes are members of a gene family in group A streptococci (GAS)that encode for antiphagocytic cell-surface proteins and/orimmunoglobulin-binding proteins. Previously sequenced genes in this familyhave been named "emm," "fcrA," "enn," "arp," "protH," and "mrp"; hereinthey will be referred to as the "emm gene family." The genes in the emmfamily are located in a cluster occupying 3-6 kb between the genes mry andscpA on the chromosome of Streptococcus pyogenes. Most GAS strains containone to three tandemly arranged copies of emm-family genes in the cluster,but the alleles within the cluster vary among different strains.Phylogenetic analysis of the conserved sequences at the 3' end of thesegenes differentiates all known members of this family into fourevolutionarily distinct emm subfamilies. As a starting point to analyze howthe different subfamilies are related evolutionarily, the structure of theemm chromosomal region was mapped in a number of diverse GAS strains byusing subfamily-specific primers in the polymerase chain reaction. Ninedistinct chromosomal patterns of the genes in the emm gene cluster werefound. These nine chromosomal patterns support a model for the evolution ofthe emm gene family in which gene duplication followed by sequencedivergence resulted in the generation of four major-gene subfamilies inthis locus.