Expression and characterization of UDPGlc dehydrogenase (KfiD), which is encoded in the type-specific region 2 of the Escherichia coli K5 capsule genes.

Region 2 of the Escherichia coli K5 capsule gene cluster contains four genes (kfiA through -D) which encode proteins involved in the synthesis of the K5 polysaccharide. A DNA fragment containing kfiD was amplified by PCR and cloned into the gene fusion vector pGEX-2T to generate a GST-KfiD fusion protein. The fusion protein was isolated from the cytoplasms of IPTG (isopropyl-beta-D-thiogalactopyranoside)-induced recombinant bacteria by affinity chromatography and cleaved with thrombin. The N-terminal amino acid sequence of the cleavage product KfiD' corresponded to the predicted amino acid sequence of KfiD with an N-terminal glycyl-seryl extension from the cleavage site of the fusion protein. Anti-KfiD antibodies obtained with KfiD' were used to isolate the intact KfiD protein from the cytoplasms of E. coli organisms overexpressing the kfiD gene. The fusion protein, its cleavage product (KfiD'), and overexpressed KfiD converted UDPGlc to UDPGlcA. The KfiD protein could thus be characterized as a UDPglucose dehydrogenase.