REGULATION OF THE FORMATE DEHYDROGENASE GENE, FDH1, IN THE METHYLOTROPHIC YEAST CANDIDA-BOIDINII AND GROWTH-CHARACTERISTICS OF AN FDH1-DISRUPTED STRAIN ON METHANOL, METHYLAMINE, AND CHOLINE

The structural gene (FDH1) coding for NAD(+)-dependent formate dehydro genase (FDH) was cloned from a genomic library of Candida boidinii, an d the FDH1 gene was disrupted in the C. boidinii genome (fdh1 Delta) b g one-step gene disruption. In a batch culture experiment, although th e fdh1 Delta strain was still able to grow on methanol, its growth was greatly inhibited and a toxic level of formate was detected in the me dium. In a methanol-limited chemostat culture at a low dilution rate ( 0.03 to 0.05 h(-1)), formate was not detected in the culture medium of the fdh1 Delta strain; however, the fdh1 Delta strain showed only one -fourth of the growth yield of the wild-type strain, Expression of FDH 1 was found to be induced by choline or methylamine (used as a nitroge n source), as well as by methanol (used as a carbon source). Induction of FDH1 was not repressed in the presence of glucose when cells were grown on methylamine, choline, or formate, and expression of FDH1 was shown to be regulated al; the mRNA level. Growth on methylamine or cho line as a nitrogen source in a batch culture was compared between the wild type and the fdh1 Delta mutant. Although the growth of the fdh1 D elta mutant was impaired and the Level of formate was higher in the fd h1 Delta mutant than in the wild-type strain, the growth defect caused by FDH1 gene disruption was small and less severe than that caused by growth on methanol. As judged from these results, the main physiologi cal role of FDH with all of the FDH1-inducing growth substrates seems to be detoxification of formate, and during growth on methanol, FDH se ems to contribute significantly to the energy yield.