Abstract
1. 1. Breakdown of [ 14C]adenine-labelled RNA, measured as the loss of radioactivity from acid-insoluble material, begins 30 min after X-irradiation of Escherichia coli and proceeds for the next 150 min at a rate proportional to the dose of irradiation. The rate of breakdown is the same for RNA synthesized before or immediately after the irradiation, while RNA synthesized 60 min later is not subjected to degradation. 2. 2. Analysis of cell-free extracts of irradiated bacteria by zonal centrifugation in sucrose density gradients shows a time-dependent decrease of radioactivity of 50-S and 30-S ribosomes. The material appears in a broad peak sedimenting between 4 S and 30 S, which decreases as the incubation progresses. 3. 3. RNA prepared by the phenol method from bacteria at various stages of post-irradiation incubation was also analysed by zonal centrifugation. The amount of 23-S and 16-S RNA was found to decrease at the same rate, while 4-S material was not degraded. Intermediate breakdown products of rRNA sedimented between 4-S and 16-S RNA. 4. 4. Final breakdown products in the incubation media were analysed by chromatography on a column of Sephadex G-25 and on paper. The radioactive breakdown products of adenine-labelled RNA formed in mineral medium were identified as AMP, GMP and oligonucleotides. In nutrient broth, hypoxanthine, AMP, oligonucleotides, and some free adenine were found.
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