1. The concentrations of folate derivatives in aerobic cultures of Saccharomyces cerevisiae (A.T.C.C. 9763) were determined by microbiological assay employing Lactobacillus casei (A.T.C.C. 7469) and Pediococcus cerevisiae (A.T.C.C. 8081). Cells cultured in media lacking l-methionine contained higher concentrations of folate derivatives than cells grown in the same media supplemented with 2.5mumol of l-methionine/ml. The concentrations of highly conjugated derivatives were also decreased by supplementing the growth medium with l-methionine. 2. DEAE-cellulose column chromatography of extracts prepared from cells grown under these conditions revealed that the concentrations of methylated tetrahydrofolates were drastically decreased by the methionine supplement. Smaller decreases were also observed in the concentrations of formylated and unsubstituted derivatives. 3. The concentrations of four enzymes of C(1) metabolism were compared after 6h of growth in the presence and in the absence of l-methionine (2.5mumol/ml). The specific activities of formyltetrahydrofolate synthetase, methylenetetrahydrofolate reductase and serine hydroxymethyltransferase were not altered by this treatment but that of 5-methyltetrahydrofolate-homocysteine methyltransferase was decreased by approx. 65% when l-methionine was supplied. The activities of 5-methyltetrahydrofolate-homocysteine methyltransferase, serine hydroxymethyltransferase and formyltetrahydrofolate synthetase were not appreciably altered by l-methionine in vitro. In contrast this amino acid was found to inhibit the activity of methylenetetrahydrofolate reductase. 4. Feeding experiments employing sodium [(14)C]formate indicated that cells grown in the presence of exogenous methionine, although having less ability to convert formate into methionine, readily incorporated (14)C into serine and the adenosyl moiety of S-adenosylmethionine. 5. It is suggested that exogenous l-methionine controls C(1) metabolism in Saccharomyces principally by regulation of methyl-group biogenesis within the folate pool.

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