Turnover of nucleic acids in a multiplying animal cell. 1. Incorporation studies

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Abstract
The incorporation of [8-C14]adenine, [2-C14]-uridylic acid and [P32]orthophosphate into the nucleic acids of HeLa cells was studied. Radioactivity entered the nuclear ribonucleic acid (RNA) much more rapidly than the cytoplasmic RNA, and incorporation into cyto-plasmic RNA usually attained its maximum rate only after 1 hour. The observed kinetics of incorporation were dependent on the concentration of radioactive precursor in the culture medium. Trace amounts of radioactive precursor were soon exhausted and incorporation into the nuclear RNA then ceased, although radioactivity continued to enter the cytoplasmic RNA for some hours. The initial lag in the incorporation of radioactivity into cytoplasmic RNA could be eliminated by a suitable choice of experimental conditions. When the intracellular pools of unlabelled precursors were diminished as far as possible, 20% of the total radioactivity in RNA was found in cytoplasmic RNA within 15 minutes. The kinetics of incorporation of radioactive adenine and uridylic acid indicated that the nuclear RNA contained three fractions that could be distinguished by the rates at which they became radioactive. A rapidly labelled fraction was selectively degraded during the phenol extraction of RNA. When the cells were labelled with [p32]orthophosphate, the specific radioactivities of the cytoplasmic RNA nucleotides obtained after alkaline hydrolysis of the RNA were all different and showed no tendency to become equal even after 6 hours labelling. The specific radioactivities of three of the nuclear RNA nucleotides were equal after only 1 hour labelling, but the uridylic acid consistently had a much higher specific radioactivity than had the other nucleotides.

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