, 2007). GlcNAc-1-phosphate transferase transfers GlcNAc-1-phosphate from undecaprenyl phosphate (UDP)-GlcNAc to the carrier, yielding C50-P-P-GlcNAc. The rhamnosyl transferase (WbbL) (Mills et

al., 2004; Grzegorzewicz et al., 2008) encoded by Rv3265c attaches the rhamnosyl residue (Rha) to C50-P-P-GlcNAc to produce C50-P-P-GlcNAc-Rha (Fig. 1b), which is then further elongated with galactan and arabinan and finally mycolylated arabinogalactan attached to the peptidoglycan. However, GlcNAc-1-phosphate transferase has not yet been identified in mycobacteria. Lipopolysaccharides found in the outer see more membrane of Gram-negative bacteria are made up of a hydrophobic lipid (lipid A), a hydrophilic core polysaccharide chain and a hydrophilic O-antigenic polysaccharide side chain (O-antigen). In most cases, O-specific chains are formed by repeating units of oligosaccharides that exhibit a strain-specific structural diversity (Reeves et al., 1996). The biosynthesis of an O repeating unit starts on the

cytosolic face of the plasma membrane with the formation of a sugar–phosphodiester linkage with a lipid carrier. After the initiation reaction, additional sugars are incorporated to complete the O unit in reactions catalyzed by specific glycosyltransferases, which are either soluble cytosolic enzymes or peripheral eltoprazine membrane proteins associated with the plasma membrane by ionic interactions (Feldman et al., 1999; Samuel & Reeves, 2003). The GlcNAc is the first Compound Library sugar of the O unit and the wecA gene (formerly called rfe) specifies the UDP-GlcNAc: undecaprenyl phosphate (Und-P) GlcNAc-1-phosphate transferase (WecA) that catalyzes the first step in the biosynthesis of O unit (Alexander & Valvano, 1994; Raetz & Whitfield, 2002; Schäffer et al., 2002). That is, WecA from Gram-negative bacteria transfers GlcNAc-1-phosphate from UDP-GlcNAc to Und-P (C55-P), forming C55-P-P-GlcNAc.

This reaction is similar to the formation of C50-P-P-GlcNAc in mycobacteria, although decaprenyl phosphate, rather than the usual Und-P, plays the central role as the carrier lipid in all known cell wall biosynthetic processes in mycobacteria (Scherman et al., 1996; Mahapatra et al., 2005; Mikušováet al., 2005). Mycobacterium tuberculosis Rv1302 shows high homology to Escherichia coli WecA protein (Amer & Valvano, 2001). Rv1302 and E. coli WecA have 28% identity (85/305) and 44% (137/305) positivity. A Mycobacterium smegmatis MSMEG_4947 ortholog was found by a blastp search using M. tuberculosis Rv1302 protein as a query; Rv1302 and MSMEG_4947 have 79% identity (301/380) and 83% positivity (316/380); and MSMEG_4947 and E. coli WecA have 29% (92/313) and 44% (138/313), respectively.