Evidence for group A-related M protein genes in human but not animal-associated group G streptococcal pathogens.

Group G streptococci have on their surface antiphagocytic M protein-like antigens. To determine if these organisms have genes similar to the M protein genes of Streptococcus pyogenes (group A), DNA from independent group G isolates of human and animal origin were tested for homology to probes representing sequences encoding the carboxy-terminus and leader peptide of the type 12 M protein (M12) of group A streptococci. All eight human-associated group G strains tested had DNA homologous to the carboxy-terminal probe. Six of these strains also had DNA that hybridized with the leader peptide probe. Using probes representing the group A M12 gene (emm12) and adjacent 5' sequences, we found that one of these strains, known to produce an M12 antigen, had a nearly complete duplication of the group A emm12 gene, differing only in 0.26 kb of sequence at the 5' end. The other human-associated strains did not hybridize with emm12-specific sequences. None of the group G strains had homology to 5' proximal sequences thought to be associated with group A emm12 regulation, but all the human-associated strains had DNA homology to a 1.5 kb DNA segment which mapped 2.5 kb upstream of the emm12 gene in group A streptococci. None of the twelve animal-associated strains tested hybridized with any of the probes used in this study. These results suggest that human but not animal-associated group G isolates have group A-related M protein genes. We propose that expression of these genes are critical for infection of the human host and that group A and G shared upstream sequences could encode additional virulence factors.