Induction and Repression in the S -Adenosylmethionine and Methionine Biosynthetic Systems of Saccharomyces cerevisiae

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Abstract
Two methionine biosynthetic enzymes and the methionine adenosyltransferase are repressed in Saccharomyces cerevisiae when grown under conditions where the intracellular levels of S -adenosylmethionine are high. The nature of the co-repressor molecule of this repression was investigated by following the intracellular levels of methionine, S -adenosylmethionine, and S -adenosylhomocysteine, as well as enzyme activities, after growth under various conditions. Under all of the conditions found to repress these enzymes, there is an accompanying induction of the S -adenosylmethionine-homocysteine methyltransferase which suggests that this enzyme may play a key role in the regulation of S -adenosylmethionine and methionine balance and synthesis. S -methylmethionine also induces the methyltransferase, but unlike S -adenosylmethionine, it does not repress the methionine adenosyltransferase or other methionine biosynthetic enzymes tested.