We have cloned the gene cluster encoding three subunits of membrane-bound gluconate dehydrogenase (GADH) from Erwinia cypripedii ATCC 29267 in Escherichia coli by performing a direct-expression assay. The positive clone converted D-gluconate to 2-keto-D-gluconate (2KDG) in the culture medium. Nucleotide sequence analysis of the GADH clone revealed that the cloned fragment contained the complete structural genes for a 68-kDa dehydrogenase subunit, a 47-kDa cytochrome c subunit, and a 24-kDa subunit of unknown function and that the genes were clustered with the same transcriptional polarity. Comparison of the deduced amino acid sequences and the NH 2 -terminal sequences determined for the purified protein indicated that the dehydrogenase, cytochrome c, and 24-kDa subunits contained typical signal peptides of 22, 19, and 42 amino acids, respectively. The molecular masses of the processed subunits deduced from the nucleotide sequences (65, 45, and 20 kDa) coincided well with the molecular masses of subunits estimated by sodium dodecyl sulfatepolyacrylamide gel electrophoresis. In E. cypripedii and recombinant E. coli, the GADH was constitutively formed and the activity of GADH was enhanced more than twofold by addition of D-gluconate to the medium. The conversion of D-glucose to D-gluconate, 2-keto-D-gluconate (2KDG), and 2,5-diketo-D-gluconate (25DKG) in several species of the genera Acetobacter, Gluconobacter, and Erwinia is mediated by membrane-bound dehydrogenases linked to the cytochrome chains located in the cytoplasmic membrane of the bacterium (1, 31). Membrane-bound gluconate dehydrogenases (GADHs), which catalyze the production of 2KDG from D-gluconate, have been purified and characterized from Pseudomonas aeruginosa (16,19), P. fluorescens (17), Klebsiella pneumoniae (17), Serratia marcescens (17), and Gluconobacter dioxyacetonicus (28). The subunit structures and enzymatic properties of these GADHs were found to be very similar (17). These GADHs consisted of three subunits; namely, flavoprotein, cytochrome c, and a third subunit whose function is unknown. Despite these findings, no further work on gene structures has been reported.In the present work, we describe the direct-expression cloning of the gene cluster encoding three subunits of membranebound GADH from Erwinia cypripedii ATCC 29267. E. coli K-12 derivatives are capable of synthesizing the apo-glucose dehydrogenase (apo-GDH) but not the cofactor, pyrroloquinoline quinone (PQQ), which is essential for the formation of the holoenzyme (3, 5). When PQQ is present in the medium, the holoenzyme is reconstituted and then E. coli is capable of oxidizing glucose to gluconate (7). It has also been reported that the expression of PQQ synthase genes in E. coli resulted in GDH activity in the absence of exogenous PQQ (13). Therefore, we tried to convert D-glucose to 2KDG via D-gluconate by using recombinant E. coli harboring the cloned GADH gene in the presence of PQQ.
MATERIALS AND METHODSBacterial strains, media, and culture conditions. E. cypripedii ATCC ...