A Novel Epimerase That Converts GlcNAc-P-P-undecaprenol to GalNAc-P-P-undecaprenol in Escherichia coli O157

Escherichia coli strain O157 produces an O-antigen with the repeating tetrasaccharide unit alpha-D-PerNAc-alpha-L-Fuc-beta-D-Glc-alpha-D-GalNAc, preassembled on undecaprenyl pyrophosphate (Und-P-P). These studies were conducted to determine whether the biosynthesis of the lipid-linked repeating tetrasaccharide was initiated by the formation of GalNAc-P-P-Und by WecA. When membrane fractions from E. coli strains K12, O157, and PR4019, a WecA-overexpressing strain, were incubated with UDP-[H-3] GalNAc, neither the enzymatic synthesis of [H-3]GlcNAcP-P-Und nor [H-3] GalNAc-P-P-Und was detected. However, when membrane fractions from strain O157 were incubated with UDP-[H-3] GlcNAc, two enzymatically labeled products were observed with the chemical and chromatographic properties of [H-3] GlcNAc-P-P-Und and [H-3] GalNAc-P-P-Und, suggesting that strain O157 contained an epimerase capable of interconverting GlcNAc-P-P-Und and GalNAc-P-P-Und. The presence of a novel epimerase was demonstrated by showing that exogenous [H-3] GlcNAc-P-P-Und was converted to [H-3] GalNAc-P-P-Und when incubated with membranes from strain O157. When strain O157 was metabolically labeled with [H-3] GlcNAc, both [H-3] GlcNAc-P-P-Und and [H-3] GalNAc-P-P-Und were detected. Transformation of E. coli strain 21546 with the Z3206 gene enabled these cells to synthesize GalNAc-P-P-Und in vivo and in vitro. The reversibility of the epimerase reaction was demonstrated by showing that [H-3] GlcNAc-P-P-Und was reformed when membranes from strain O157 were incubated with exogenous [H-3] GalNAc-P-P-Und. The inability of Z3206 to complement the loss of the gne gene in the expression of the Campylobacter jejuni N-glycosylation system in E. coli indicated that it does not function as a UDP-GlcNAc/UDPGalNAc epimerase. Based on these results, GalNAc-P-P-Und is synthesized reversibly by a novel GlcNAc-P-P-Und epimerase after the formation of GlcNAc-P-P-Und by WecA in E. coli O157.