TRANSFORMATION SYSTEM FOR AN ASPOROGENOUS METHYLOTROPHIC YEAST, CANDIDA-BOIDINII - CLONING OF THE OROTIDINE-5'-PHOSPHATE DECARBOXYLASE GENE (URA3), ISOLATION OF URACIL AUXOTROPHIC MUTANTS, AND USE OF THE MUTANTS FOR INTEGRATIVE TRANSFORMATION

An integrative transformation system was established for an asporogenous methylotrophic yeast, Candida boidinii. This system uses a uracil auxotrophic mutant of C. boidinii as the host strain in combination with its URA3 gene as the selectable marker. First, the C. boidinii URA3 gene coding for orotidine-5'-phosphate decarboxylase (ODCase) was cloned by using complementation of the pyrF mutation of Escherichia coli. Next, the host ODCase-negative mutant strains (ura3 strains) were isolated by mutagenesis and selection for 5-fluoro-orotic acid (5-FOA) resistance. Five ura3 host strains that exhibited both a low reversion rate and good methylotrophic growth were obtained. All of these strains could be transformed to Ura+ phenotype with a C. boidinii URA3-harboring plasmid linearized within the Candida DNA. The transformants had a stable Ura+ phenotype after nonselective growth for 10 generations. These results and extensive Southern analysis indicated that the linearized plasmid was integrated into the host chromosomal DNA by homologous recombination at the URA3 locus in C. boidinii.