CHARACTERIZATION OF A P1-DEFICIENT STRAIN OF STREPTOCOCCUS-MUTANS THAT EXPRESSES THE SPAA PROTEIN OF STREPTOCOCCUS-SOBRINUS

The Streptococcus sobrinus SpaA protein and the Streptococcus mutans P 1 protein share 66% sequence homology at the amino acid level. To dete rmine if the SpaA protein can be expressed in S. mutans and functional ly replace the P1 protein, the spaA gene of S, sobrinus 6715 was isola ted from plasmid pXI303 and inserted into the Escherichia coli-Strepto coccus shuttle vector pVA838. The resulting plasmid pXI600 was transfo rmed into the P1-deficient strain S. mutans 834 that has defects in sa liva-mediated aggregation and In the ability to adhere to saliva-coate d hydroxyapatite surfaces. Western blot (immunoblot) analysis of cellu lar protein fractions of S. mutans 834(pXI600) detected in mutanolysin -solubilized cell walls a major protein of 210 kDa with an electrophor etic mobility similar to that of S. sobrinus SpaA protein and a minor 210-kDa protein and a major 64-kDa protein in the extracellular protei n fraction. Analysis of virulence traits showed that expression of Spa A protein by S. mutans 834(pXI600) cells had restored the ability of t he S. mutans 834 cells to aggregate in the presence of saliva or saliv ary agglutinin but not to adhere to saliva-coated hydroxyapatite. This cell aggregation was inhibited specifically by antisera to S. sobrinu s SpaA protein. These results indicate that SpaA plays a role in the v irulence of S. sobrinus by specifically interacting with fluid-phase s alivary agglutinin to mediate cell aggregation.