FIG 2.

Colanic acid genes and the CA biosynthetic pathway. (A) Genes of the CA operon. The promoter region contains a binding sequence that induces gene expression when occupied by the RcsAB heterodimer. (B) Selected substrates and enzymes in the CA biosynthesis and transport pathway (29, 39, 58). (Adapted from reference 34 with permission of the publisher.) Enzymes are devoted to precursor synthesis (CpsG, CpsG, Gmd, WcaG, and WcaH) or to CA assembly and transport (WcaJ, WcaA, WcaC, WcaE, WcaI, WcaL, WcaB, WcaF, WzxC, WcaD, Wzc, Wzb, and Wza). Undecaprenyl pyrophosphate synthase (UppS) catalyzes the condensation reaction of isopentenyl diphosphate and farnesyl diphosphate to generate undecaprenyl pyrophosphate (Und-PP). Undecaprenyl pyrophosphate phosphatase (UppP) converts Und-PP to undecaprenyl phosphate (Und-P). WcaJ, a glycosyl transferase, initiates the first dedicated step of CA assembly by adding glucose-1-phosphate from UDP-glucose (UDP-Glu) to Und-P, generating Und-PP-Glu. Additional sugar residues are added stepwise to Und-PP-Glu: GDP-fucose (GDP-Fuc), UDP-galactose (UDP-Gal), and UDP-glucaronic acid (UDP-GlcA). WzxC, a polysaccharide transporter, flips the lipid-linked CA repeat unit to the periplasmic face of the inner membrane (IM). WcaD polymerizes the repeat units to create CA polymers, and the Wzc-Wza complex transports mature CA through the outer membrane (OM) and out of the cell. OAc, O-acetyl; PG, peptidoglycan; Pyr, pyruvate.