Unraveling the function of glycosyltransferases in Streptococcus thermophilus Sfi6

• Published in: Journal of Bacteriology Vol. 181; no. 20; pp. 6354 - 6360
• Main Authors: Stingele, F, Newell, J.W, Neeser, J.R
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 01.10.1999
• Subjects: Amino Acid Sequence; carbohydrate metabolism; chemical composition; enzyme activity; Enzymes and Proteins; EpsE protein; epsF gene; EpsF protein; EpsG protein; epsI gene; exopolysaccharides; Galactosyltransferases; Genes, Bacterial; glycosyltransferase; Glycosyltransferases - genetics; Glycosyltransferases - metabolism; hexosyltransferases; Molecular Sequence Data; Multigene Family; N-Acetylgalactosaminyltransferases; phosphogalactosyltransferase; polysaccharides; Polysaccharides, Bacterial - biosynthesis; Protein Conformation; Sequence Homology, Amino Acid; Streptococcus - enzymology; Streptococcus - genetics; Streptococcus thermophilus; Substrate Specificity
• ISSN: 0021-9193; 1098-5530; 1067-8832
• DOI: 10.1128/JB.181.20.6354-6360.1999

Streptococcus thermophilus Sfi6 produces a texturizing exopolysaccharide (EPS) consisting of a leads to 3)[alpha-D-Galp-(1 leads to 6)]-beta-D-Glcp-(1 leads to 3)-alpha-D-GalpNAc-(1 leads to 3)-beta-D-Galp-(1 leads to repeating unit. We previously identified and analyzed a 14.5-kb gene cluster from S. thermophilus Sfi6 consisting of 13 genes responsible for its EPS production. Within this gene cluster, we found a central region of genes (epsE, epsF, epsG, and epsI) that showed similarity to glycosyltransferases. In this study, we investigated the sugar specificity of these enzymes. EpsE catalyzes the first step in the biosynthesis of the EPS repeating unit. It exhibits phosphogalactosyltransferase activity and transfers galactose onto the lipophilic carrier. The second step is fulfilled by EpsG, which transfers an alpha-N-acetylgalactosamine onto the first beta-galactoside. The activity of EpsF was determined by characterizing the EPS produced by an S. thermophilus epsF deletion mutant. This EPS consisted of the monosaccharides Gal, Glc, and GalNAc in an approximately equimolar ratio, thus suggesting that epsF codes for the branching galactosyltransferase. epsI probably codes for the beta-1,3-glucosyltransferase, since it is the only glycosyltransferase to which no gene has been assigned and it exhibits similarity to other beta-glycosyltransferases. EpsE shows the conserved features of phosphoglycosyltransferases, whereas EpsF and EpsG exhibit the primary structure of alpha-glycosyltransferases, belonging to glycosyltransferase family 4, whose members are conserved in all major phylogenetic lineages, including the Archaea and Eukaryota.