The effects of inhibitors of RNA and DNA synthesis on protein synthesis and polysome levels in mouse L-cells.

Abstract

AbstractMetabolic inhibitors of RNA synthesis, such as actinomycin D and MPB (2‐mercapto‐1‐(β‐4‐pyridethyl) benzimidazole) have often been used to test the possibility that transcription is required for some cellular process or to measure the „half‐life”︁ of messenger RNA (mRNA). The basic assumption of this work has been that the primary effect of these inhibitors is on transcription alone, and that any effect on translation is secondary to the inhibition of mRNA synthesis. This assumption has been tested by examining the effects of these inhibitors at different doses on various parameters of the transcriptional and translation processes in mouse L‐cells in culture, i.e., the rates of RNA, DNA, and protein synthesis; the level and size of cytoplasmic polysomes; the rates of polypeptide elongation and termination, and the role of peptide chain initiation in protein synthesis. The results indicate that the basic assumption is not correct since these inhibitors affect protein synthesis by inhibiting the rate of initiation and not the level of mRNA. Another implication of the experiments is that the average mRNA in L‐cells has a half‐life of at least one cell generation (16–18 hours), and that earlier values from experiments using actinomycin are underestimates. Cordycepin (3′‐deoxyadenosine) which inhibits the production of ribosomal and messenger RNA also affects protein synthesis at the level of initiation. In contrast, two inhibitors of DNA synthesis (hydroxyurea and cytosine arabinoside) inhibit protein synthesis and the level of polysomes by some other mechanism.