Phylogenetic Analyses of the Water-Insoluble Glucan Synthesizing Glucosyltransferase Gene of Streptococcus ratti

Abstract

The nucleotide sequence of the glucosyltransferase (GTF) gene, which encodes a GTF enzyme that synthesizes a water-insoluble glucan (WIG), was determined in Streptococcus ratti FA-1 (GTC 00245^T). The gtf of S. ratti consisted of 4,350-bp that encoded for a 1,449 amino acid protein with a molecular weight of approximately 159.9 kDa and was revealed to be a gtfL type gene, which were first found in Streptococcus salivarius ATCC 25975. The deduced 35 amino acid sequence of the N-terminal was thought to be a signal peptide required for the secretion of GTF-L in S. ratti as it showed high similarity to a known GTF-L from S. salivarius. In addition, three major functional domains of GTF : an N-terminal variable region, a conserved catalytic site for sucrase activity, and C-terminal YG repeating units for glucan binding were also found in GTF-L from S. ratti. The percentage homology of the GTF-L amino acid sequences from S. ratti and S. salivarius was 99.7%. Although GTF-L were classified into the WIG-synthesizing GTF group, phylogenetic analysis suggested that GTF-L were positioned outside of the WIG-synthesizing GTF group of mutans streptococci.