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Turnover of 45 S RNA of Regenerating Liver and Walker Tumor¹

Department of Pharmacology, Baylor University College of Medicine, Houston, Texas

The rate of disappearance of the 45 S peak in the sucrose density gradient sedimentation profile of nuclear and nucleolar RNA of regenerating liver of rats treated with actinomycin D shows that the average half-lives of 45 S RNA of the nuclei and nucleoli are 8 and 4.7 min, respectively. Similar determinations based on the rate of transfer of radioactivity from the 45 S peaks to more slowly sedimenting RNA peaks indicate that the average half-lives of nuclear and nucleolar RNA of regenerating liver are 4.1 and 3.7 min, respectively. These results indicate that the nucleolar RNA in regenerating liver turns over at a faster rate than the whole nuclear RNA.

The rates of disappearance of the 45 S peak and that of the radioactivity in the 45 S peak of nucleolar RNA of Walker tumor cells after actinomycin D treatment show that the average halflife of 45 S RNA of the tumor nucleoli is 6.8 min. Calculations from these data and the pool sizes of 45 S RNA in nucleoli indicate that the rates of synthesis of 45 S RNA in normal liver nucleoli, nucleoli of regenerating liver, and the nucleoli of the Walker tumor are 4, 16, and 32 femtograms (10^-15 gm)/min, respectively.

¹ These studies are supported in part by grants from the National Science Foundation, the American Cancer Society, the Jane Coffin Childs Fund and the USPHS (CA 08182).

Received 4/11/66. Accepted 7/21/66.