Functional Characterization of Two Polymerizing Glycosyltransferases for the Addition of -Acetyl-d-galactosamine to the Capsular Polysaccharide of .

The exterior surface of the human pathogen is coated with a capsular polysaccharide (CPS) that consists of a repeating sequence of 2-5 different sugars that can be modified with various molecular decorations. In the HS:2 serotype from strain NCTC 11168, the repeating unit within the CPS is composed of d-ribose, -acetyl-d-galactosamine, and a d-glucuronic acid that is further amidated with either serinol or ethanolamine. The d-glucuronic acid moiety is also decorated with d-glycero-l-gluco-heptose. Here, we show that two different GT2 glycosyltransferases catalyze the transfer of -acetyl-d-galactosamine from UDP-NAc-d-galactosamine furanoside to the C4-hydroxyl group of the d-glucuronamide moiety at the growing end of the capsular polysaccharide chain. Catalytic activity was not observed with glycosides of d-glucuronic acid, and thus, the C6-carboxylate of the d-glucuronic acid moiety must be amidated prior to chain elongation. One of these enzymes comprises the N-terminal domain of Cj1438 (residues 1-325) and the other is from the N-terminal domain of Cj1434 (residues 1-327). These two glycosyltransferases are ∼87% identical in sequence, but it is not clear why there are two glycosyltransferases from the same gene cluster that apparently catalyze the same reaction. This discovery represents the second polymerizing glycosyltransferase that has been isolated and functionally characterized for the biosynthesis of the capsular polysaccharide in the HS:2 serotype of .