Abstract
The activities of the enzymes involved in thiamin biosynthesis, thiamin transport system and thiamin-repressible acid phosphatase in Saccharomyces cerevisiae were coordinately repressed by exogenous thiamin .We identified a strain carrying a recessive constitutive mutation (thi80) with these activities in thiamin metabolism. The thi80 mutant shows markedly reduced activity of thiamin pyrophosphokinase (EC 2.7.6.2) and an increase of the intracellular thiamin pyrophosphate content of the mutant cells grown with exogenous thiamin was appreciably low compared to that of the wild-type strain. These results suggest that a negative regulatory mechanism by thiamin pyrophsphate as an effector participates in the control of thiamin metabolism .On the other hand, we have obtained two thiamin auxotrophic mutants carrying recessive mutations which lack the positive regulatory genes, THI2(PHO6) and THI3 for expression of thiamin metabolism. The THI3 gene differs in the regulation of thiamin metabolism. The THI3 gene differs in the regulation of thiamin transport from the THI2(PHO6) gene. We have further isolated plasmid clones from an S. cerevisiae genomic library which complement thiamin auxotrophy in these mutants. Complementations of the activities for thiamin metabolism in these mutants transformed by some plasmids with THI2(PHO6) and THI3 gene were also examined .We propose a working hypothesis that thiamin metabolism in S. cerevisiae is controlled by the positive factors, THI2(PHO6) and THI3, whose actions are regulated negatively by the intracellular thiamin pyrophosphate level.
