MOLECULAR EVOLUTION OF A MULTIGENE FAMILY IN GROUP-A STREPTOCOCCI

The emm genes are members of a gene family in group A streptococci (GA S) that encode for antiphagocytic cell-surface proteins and/or immunog lobulin-binding proteins. Previously sequenced genes in this family ha ve been named ''emm, ''fcrA, ''enn, ''arp, ''protH, '' and ''mrp'', he rein they will be referred to as the ''emm gene family.'' The genes in the emm family are located in a cluster occupying 3-6 kb between the genes mry and scpA on the chromosome of Streptococcus pyogenes. Most G AS strains contain one to three tandemly arranged copies of emm-family genes in the cluster, but the alleles within the cluster vary among d ifferent strains. Phylogenetic analysis of the conserved sequences at the 3' end of these genes differentiates all known members of this fam ily into four evolutionarily distinct emm subfamilies. As a starting p oint to analyze how the different subfamilies are related evolutionari ly, the structure of the emm chromosomal region was mapped in a number of diverse GAS strains by using subfamily-specific primers in the pol ymerase chain reaction. Nine distinct chromosomal patterns of the gene s in the emm gene cluster were found. These nine chromosomal patterns support a model for the evolution of the emm gene family in which gene duplication followed by sequence divergence resulted in the generatio n of four major-gene subfamilies at this locus.