Molecular basis of type 2Gn RPS biosynthesis and structure. (A) The operon for type 2Gn RPS biosynthesis in S. gordonii 38 consists of 14 genes located between nrdG and orfO. The first four of these encode regulatory proteins. The remaining 10 genes include seven for putative glycosyltransferases, wzx and wzy for the transport and polymerization of RPS repeating units and glf for the epimerase that supplies UDP-Galf, one of five essential RPS precursors. (B) The deduced roles of the seven glycosylytransferases encoded by the type 2Gn RPS gene cluster in synthesis of the corresponding lipid-linked heptasaccharide repeating unit. The β1–6 linkage between Glc and Galf in type 2Gn RPS is formed by Wzy-dependent polymerization of RPS repeating units.