Synthesis of ribonucleic acid in baby-hamster kidney cells in the presence of actinomycin D.

1. RNA molecules with sedimentation values in sucrose gradients of 12-20s are synthesized in baby-hamster kidney cells even after prolonged incubation in medium containing 1mug. of actinomycin D/ml. 2. The rate of formation of this RNA is dependent on the age of the cultures and is greatest during the exponential phase of growth. 3. Growth of cells on nutritionally poor medium causes degradation and inhibits the synthesis of these RNA fractions. 4. Replacement of the nutritionally poor medium with a rich medium stimulates the synthesis of actinomycin-resistant RNA. This stimulation is blocked by cycloheximide. 5. The base composition of this RNA is characterized by low cytidine and high guanosine values.