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Journal of Bacteriology, October 1999, p. 6354-6360, Vol. 181, No. 20
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Unraveling the Function of Glycosyltransferases in Streptococcus thermophilus Sfi6

Francesca Stingele,* John W. Newell, and Jean-Richard Neeser

Nestlé Research Center, Nestec Ltd., Vers-chez-les-Blanc, 1000 Lausanne 26, Switzerland

Received 3 May 1999/Accepted 26 July 1999

Streptococcus thermophilus Sfi6 produces a texturizing exopolysaccharide (EPS) consisting of a right-arrow3)[alpha -D-Galp-(1right-arrow6)]-beta -D-Glcp-(1right-arrow3)-alpha -D-GalpNAc-(1right-arrow3)-beta -D-Galp-(1right-arrow 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.

* Corresponding author. Mailing address: Nestlé Research Center, Nestec Ltd., P.O. Box 44, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland. Phone: 41-21-7858923. Fax: 41-21-7858925. E-mail: francesca.stingele{at}rdls.nestle.com.

Journal of Bacteriology, October 1999, p. 6354-6360, Vol. 181, No. 20
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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