Fig. 1: Schematic representation of the S. mutans rhamnopolysaccharide biosynthetic gene clusters and the proposed mechanism of SCC modification with the Glc side-chains and GroP.
From: Structure and mechanism of biosynthesis of Streptococcus mutans cell wall polysaccharide
A Schematic representation of the genetic loci encoding enzymes involved in the biosynthesis of rhamnopolysaccharides expressed by S. mutans serotypes c, k, f, and e: Serotype c Carbohydrate (SCC), Serotype k Carbohydrate (SKS), Serotype f Carbohydrate (SFC) and Serotype e Carbohydrate (SEC), respectively. Genes in Region 1 participate in the synthesis of the polyrhamnose backbone. Genes in Region 2 participate in modification of the polyrhamnose backbone with the side-chains and GroP. The SCC gene cluster contains genes designated sccABCDEFGHMNPQ corresponding to smu.824-835 in S. mutans UA159 (GenBank: AE014133.2). The SKC gene cluster represents the SMULJ23_1190-SMULJ23_1180 gene locus in S. mutans LJ23 (GenBank: AP012336.1). The SFC gene cluster represents the K2F51_02855-K2F51_02900 gene locus in S. mutans OMZ175 (GenBank: CP082153.1). The SEC gene cluster represents the CO204_06335-CO204_06385 gene locus in S. mutans LAR01 (GenBank: CP023477.1). The percentage of amino acid identity to proteins encoded by the SCC gene cluster are noted below each gene. B Model illustrating SCC side-chain assembly and transfer to peptidoglycan in S. mutans serotype c. Side-chain addition starts with the synthesis of Glc-P-Und, on the cytosolic surface of the plasma membrane, catalyzed by two independent Glc-P-Und synthases (SccN and SccP) using UDP-Glc and Und-P as substrates. Following translocation of Glc-P-Und to the periplasmic surface (mediated by currently unidentified flippase proteins, indicated as a question mark), the GT-C type transferases, SccM and SccQ, transfer Glc from Glc-P-Und to the polyrhamnose backbone at the 2- and 4-positions of 3-Rha components, respectively. In addition, transfer of Glc from Glc-P-Und to the 4-position of 2-Rha takes place, but only to a limited extent (glycosyltransferase unknown). Following glucosylation, SccH transfers GroP from phosphatidylglycerol to the 6-OH of Glc residues found only at the 2-position of 3-Rha. After side-chain assembly has been completed, SCC is transferred to peptidoglycan by LytR-Cps2A-Psr (LCP) family proteins.
