MOLECULAR POPULATION GENETIC-ANALYSIS OF THE ENN SUBDIVISION OF GROUP-A STREPTOCOCCAL EMM-LIKE GENES - HORIZONTAL GENE-TRANSFER AND RESTRICTED VARIATION AMONG ENN GENES

The group A streptococcal emm-like genes, which encode the cell-surfac e M and M-like proteins, are divided into distinct mrp, emm and enn su bdivisions and are clustered together in a region of the chromosome ca lled the vir regulon, In order to understand the mechanisms involved i n the evolution of emm-like genes, a 180 bp fragment of the 5' variabl e region of the enn gene was characterized in 31 strains for which emm sequences and multilocus enzyme electrophoretic profiles have been pr eviously determined, The results demonstrate that nucleotide polymorph isms at the enn locus are generated predominantly by point mutations a nd short deletions or insertions, and that variation among enn and emm genes has arisen by similar mechanisms. However, diversity at the enn locus is restricted in comparison to the emm locus, Moreover, there i s strong evidence for intragenic recombination at the enn locus and th e pattern of distribution of emm and enn alleles among strains suggest s that these genes may be independently acquired by horizontal transfe r and recombination from distinct donor strains, thereby generating a mosaic structure for the vir regulon, The results add to a growing bod y of evidence that horizontal gene transfer has played a major role in the evolution of Streptococcus pyogenes vir regulons.