Cloning and nucleotide sequence of type 3 M protein gene (emm3) consisting of an N-terminal variable portion and C-terminal conserved C repeat regions: relation to other genes of Streptococcus pyogenes. Streptococcal Diseases Study Group.

The structural gene for type 3 M protein of Streptococcus pyogenes, which consists of an N-terminal variable portion and C-terminal conserved repeat regions, has been cloned by the polymerase chain reaction (PCR) with two primers (K-1 and K-2). They were selected from the best conserved region of the leader sequences and of the C-terminal portion near the Hexapeptide (LPSTGE) sequence found in different M proteins. From the nucleotide sequence of the product, 1645 base pairs were determined, including 32 amino acids of the leader sequences, the complete N-terminal variable region and the conserved C repeat region. Analysis of the deduced amino acids of the sequence revealed the existence of two major repeat regions, the B and C repeat regions. Comparison of the C-repeat regions among M3 and other M proteins showed them to be more than 90% identical. The two B repeat blocks in M3 protein are also similar to those in M12 protein. Predictive secondary structure analysis of M3 protein reveals a strong alpha-helical potential. The algorithm also shows that the beta-sheet and turn potential for region 23-42 in M3 protein are similar to those for region 28-50 in M12 protein. The results indicate that M3 protein is closely related to M12 protein.