The emm gene of group A streptococci exhibits a unique type of molecular evolution. One half of the gene is genetically diverse and the other half is strictly conserved when different alleles of the gene are compared. The divergent amino-terminal half of the gene is characterized by a series of tandem repeats of 21 to 105 base pairs. The sequence and number of the repeat units are sites of observed diversity. These sequence differences are translated into antigenic differences in the M protein. To discover the molecular mechanisms of emm genetic diversity, isogenic, antigenic variants will be isolated on the basis of their immunological diversity. A simultaneous positive and negative selection using two monoclonal antibodies that recognize different antigenic determinants on the same M molecule will be used. The genetic changes will be determined by sequence analysis. These findings will provide a measure of the type(s) of gene changes that result in antigenic diversity. Protein fusions which fuse the variable repeats into an easily selectable marker gene will be constructed. The fusions will be used to measure recombination frequencies among tandem repeats in both E. coli and group A streptococci, and to begin to identify the enzymatic pathway leading to variation.
