Regulation of the Biosynthesis of Thiamine by Yeast
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Repression of the enzymes involved in the biosynthesis of thiamine and the phosphate ester of 2 methyl-4-amino-5-hydroxymethylpyrimidine was found to the folowing approximate degree: (1) a two-fold repression at 1.5 mg. of thiamine/liter, increasing to a 6-fold repression at 10 mg. of thiamine/liter of the enzyme "thiamine synthase;" (2) a two-fold repressino at 3 mg. of thiamine/liter increasing to complete repression at 9 and 10 mg. of thiamine/liter of the enzyme "pyrimidine kinase." The products of the reaction of the 5-hydroxymethylpyrimidine to its monophosphate ester, and the coupling reaction of the pyrimidine and thiazole moieties to give thiamine were identified by chromatographic, bioautographic and microbiological techniques. The enzymes tested were from cell-free extracts of the yeast cell Saccharomyces cerivisiae. The amounts of thiamine mentioned above refer to the quanitity added per one liter of basal growth medium while growing this organism.