Streptococcus salivarius ATCC 25975 possesses at least two genes coding for primer-independent glucosyltransferases

Fractionation of the culture medium showed that Streptococcus salivarius ATCC 25975 secreted a glucosyltransferase (Gtf) that was primer independent. On the basis of this observation, a gene library of S. salivarius chromosomal DNA cloned into lambda L47.1 was screened for a gene(s) coding for such an activity. As a result of this screening process, two new gtf genes, gtfL and gtfM, both of which coded for primer-independent Gtf activities, were isolated. GtfL produced an insoluble glucan that was refractory to digestion by the endo-(1-->6)-alpha-D-glucanase. of Chaetonium gracile, while GtfM produced a soluble glucan that was readily degraded by the glucanase. Comparison of the deduced amino acid sequences of gtfL and gtfM with 10 other available Gtf sequences allowed the relatedness of the conserved catalytic regions to be assessed. This analysis showed that the 12 enzymes did not form clusters based on their primer dependencies or on their product solubilities. Further analysis of the YG repeats in the C-terminal glucan-binding domains of GtfJ, GtfK, GtfL, and GtfM from S. salivarius showed that there was strong homology between a block of contiguous triplet YG repeats present in the four alleles. These blocks of YG repeats were coded for by a region of each gene that appeared to have arisen as a result of a recent duplication event(s).