Enzymatic synthesis of lipopolysaccharide in Escherichia coli -: Purification and properties of heptosyltransferase 1

Heptosyltransferase I, encoded by the rfaC(waaC) gene of Escherichia coli, is thought to add L-glycero-D-manno-heptose to the inner 3-deoxy-D-manno-octulosonic acid (Kdo) residue of the lipopolysaccharide core. Lipopolysaccharide isolated from mutants defective in rfaC lack heptose and all other sugars distal to heptose. The putative donor, ADP-L-glycero-D-manno-heptose, has never been fully characterized and is not readily available, In cell extracts, the analog ADP-mannose can serve as an alternative donor for RfaC-catalyzed glycosylation of the acceptor, Kdo(2)-lipid IVA. Using a T7 promoter construct that overexpresses RfaC similar to 15,000-fold, the enzyme has been purified to near homogeneity. NH2-terminal sequencing confirms that the purified enzyme is the rfaC gene product. The subunit molecular mass is 36 kDa, Enzymatic activity is dependent upon the presence of Triton X-100 and is maximal at pH 7.5. The apparent K-m (determined at near saturating concentrations of the second substrate) is 1.5 mM for ADP-mannose and 4.5 mu M for Kdo(2)-lipid IVA. Chemical hydrolysis of the RfaC reaction product at 100 degrees C in the presence of sodium acetate and 1% sodium dodecyl sulfate generates fragments consistent with the inner Kdo residue of Kdo(2)-lipid IVA as the site of mannosylation, The analog, Kdo-lipid IVA, functions as an acceptor, but is mannosylated at less than 1% the rate of Kdo(2)-lipid IVA. The purified enzyme displays no activity with ADP-glucose, GDP-mannose, UDP-glucose, or UDP-galactose. Mannosylation of Kdo(2)-lipid IVA catalyzed by RfaC proceeds in high yield and may be useful for the synthesis of lipopolysaccharide analogs. Pure RfaC can also be used together with Kdo(2)-[4'P-32]lipid IVA to assay for the physiological donor (presumably ADP-L-glycero-D-manno-heptose) in a crude, low molecular weight fraction isolated from wild type cells.