Cloning and characteristics of a positive regulatory gene, THI2 (PHO6), of thiamin biosynthesis in Saccharomyces cerevisiae.

A thi2(pho6) mutant of Saccharomyces cerevisiae, defective in the expression of structural genes for thiamin-repressible acid phosphatase and enzymes involved in thiamin biosynthesis, was found to retain sufficient thiamin transport activity. The transport activity was repressed by thiamin in growth medium. We isolated from a S. cerevisiae genomic library two hybrid plasmids, pTSR1 and pTSR2, containing 10.2- and 12.0-kilobase (kb) DNA fragments, respectively, which complement the thi2(pho6) mutation of S. cerevisiae. This gene was localized on a 6.0-kb ClaI-ClaI fragment in the subclone pTSR3. Complementation of the enzyme activities for thiamin metabolism in the thi2(pho6) mutant transformed by some plasmids with the TH12(PHO6) gene was also examined.