Allelic polymorphism of emm loci provides evidence for horizontal gene spread in group A streptococci.

Group A streptococci have a virulence regulon containing a single emm locus or two or three distinct and adjacent loci of structurally related emm family genes. The products of the emm gene cluster consist of fibrillar surface proteins, at least some of which are known to contain determinants of type specificity located in their NH2-terminal regions, lying distal to the cell surface. The emm genes can be categorized into four major subfamilies (SFs), based on structural differences within their 3' regions encoding the peptidoglycan-spanning domain. In this study, we investigate the polymorphism within the 5' region of SF-4 and SF-3 emm genes (which occupy the first and last emm positions of the gene cluster, respectively) in 22 strains representing different serotypes. Our findings indicate that unlike the centrally positioned SF-1 or SF-2 genes, SF-3 and SF-4 genes each display only limited polymorphism in their 5' regions, suggesting that their gene products may not be major contributors to type specificity. Two forms of the SF-3 gene (SF3a, SF3b) and two forms of the SF-4 gene (SF4a, SF4b) are found to exist in all four possible combinations (SF3aSF4a, SF3aSF4b, SF3bSF4a, SF3bSF4b), strongly suggesting that horizontal gene spread has contributed to the evolution of emm genes and to the generation of emm gene diversity in group A streptococci.