When cycloheximide is added to (B12)-deficient cultures before or after replenishment of the cells with B12, reversion of these cells is inhibited. This inhibition is not caused by interference of the inhibitor in the uptake of B12 as measured by division kinetics. Cycloheximide does not inhibit the initial increase in the rate of DNA synthesis caused by B12 replenishment, but within 30–45 min the rate decreases and DNA synthesis ceases. Cycloheximide added to replenished deficient cells after completion of DNA duplication inhibits cell division. The total cellular protein and RNA in replenished cells treated with cycloheximide does not change. B12 added to deficient cells does not stimulate the incorporation of [14C]leucine into protein during resumption and completion of DNA duplication. However, there is a large increase in [14C]leucine incorporation into the protein of these cells soon after completion of DNA duplication and before resumption of cell division. The addition of cycloheximide to B12-replenished or to nonreplenished deficient cells rapidly inhibits the incorporation. We suggest that the addition of B12 accelerates the rate of DNA synthesis in the deficient cells and that possibly no new protein synthesis is required except for mitosis. However, protein synthesis is needed for continuous DNA synthesis.

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