Structure and mechanism of biosynthesis ofStreptococcus mutanscell wall polysaccharide

Abstract

Streptococcus mutans, the causative agent of human dental caries, expresses a cell wall attached Serotype c-specific Carbohydrate (SCC) that is critical for cell viability. SCC consists of a repeating →3)α-Rha(1→2)α-Rha(1→ polyrhamnose backbone, with glucose (Glc) side-chains and glycerol phosphate (GroP) decorations. This study reveals that SCC has one major and two minor Glc modifications. The major Glc modification, @[alpha]-Glc, attached to position 2 of 3-rhamnose, is installed by SccN and SccM glycosyltransferases and is the site of the GroP addition. The minor Glc modifications are @[beta]-Glc linked to position 4 of 3-rhamnose installed by SccP and SccQ glycosyltransferases, and @[alpha]-Glc attached to position 4 of 2- rhamnose installed by SccN working in tandem with an unknown enzyme. Both the major and the minor @[beta]-Glc modifications control bacterial morphology, but only the GroP and major Glc modifications are critical for biofilm formation.