Recently, we found a novel primer-independent, water-soluble glucan synthase as a fourth glucosyltransferase (GTF) in a culture supernatant of strain AHT-k of Streptococcus sobrinus (Y. Yamashita, N. Hanada, and T. Takehara, Biochem. Biophys. Res. Commun. 150:687-693, 1988). In the present study, four kinds of purified GTFs, including the novel GTF, were prepared. They were composed of two primer-dependent GTFs and two primer-independent GTFs. Of the primer-dependent GTFs, one was a water-insoluble glucan synthase and the other was a water-soluble glucan synthase; both of the primer-independent GTFs were water-soluble glucan synthases (GTF-Sis). Using antisera against four purified GTFs, we concluded that the immunological properties of each were completely different from those of the others. Additionally, it was shown that the novel GTF-Si, which was previously shown to have a molecular weight of 137,000, was proteolytically degraded and could be isolated at a molecular weight of 152,000 and that Streptococcus cricetus secreted an enzyme that immunologically cross-reacted with GTF-Si. While the product of the novel GTF-Si was not an effective primer for both of the primer-dependent enzymes (water-soluble and -insoluble glucan synthases), the product of the enzyme affected the molecular size of the products of the other GTF-Sis.Glucosyltransferase (GTF) is one of most important virulence factors of Streptococcus sobrinus, one of the cariogenic bacteria (5, 14). Although several investigators have found that S. sobrinus secretes only three kinds of GTF (9, 13, 19), we have reported that strain AHT-k of S. sobrinus secretes four kinds of GTF (20). These types were separated by DEAE-cellulose column chromatography; and the four GTFs were then designated P1, P2, P3, and P4 by their order of elution from the DEAE-cellulose column. We have previously purified P1 and P3 and have observed that P1 is a primer-independent, water-soluble glucan synthase (GTFSi) (8) and that P3 is a primer-dependent, water-insoluble glucan synthase (GTF-Id) (7). P4 is thought to be a primerdependent enzyme (GTF-Sd) which synthesizes highbranched, water-soluble glucan, as described previously (18). Recently, using preparative isoelectric focusing, we purified a novel GTF-Si from S. sobrinus at a molecular weight of 137,000 which corresponded to the P2 activity (23). In this report, P2 (the novel GTF-Si) is designated GTF-SiN to distinguish it from the other GTF-Si. The immunological relationships among the four GTFs were not identified. Through a new purification procedure by immunoabsorbent column chromatography with an antibody against GTF-Id for eliminating GTF-Id from the GTF-SiN fraction, we purified an enzyme with a molecular weight of 152,000 which was immunologically and enzymologically identical to the previously purified GTF-SiN.In this report we discuss the meaning of the two forms of GTF-SiN and confirm the immunological distinctions among the four GTFs. Furthermore, we provide results of our studies of the priming effectiveness...