Abstract
1. The concentrations of RNA, DNA and protein are decreased in cells of Euglena gracilis var. bacillaris grown on suboptimum concentrations of vitamin B(12). 2. The addition of vitamin B(12) to deficient cells stimulates the incorporation of [(14)C]formate into the above cell components as well as into thymine of DNA and serine and methionine of protein. 3. In a cell-free system from vitamin B(12)-deficient cells, the incorporation of labelled formate into thymidylate is decreased to a greater extent with uridine than with deoxyuridine as the substrate. 4. The addition of unlabelled glutamate dilutes the radioactivity incorporated into thymine from labelled formate. 5. These results are interpreted to mean that, in DNA synthesis, vitamin B(12) has a greater role in the reduction of ribotides to deoxyribotides than in the reduction of formate to thymine methyl and that the vitamin B(12)-dependent conversion of glutamate into beta-methylaspartate also contributes to thymine synthesis.