Observations on nuclear RNA labeled with tritiated cytidine in human cancer cells in culture (Hela)

Abstract

Studies on nucleic acid metabolism on a single cell level with the autoradiographic technique has led in various laboratories to observations which indicate that the nuclear ribonucleic acid (RNA) serves as precursor for cytoplasmic RNA^1_6. Labeled RNA precursors, as H³-cytidine were first incorporated in the cell nucleus (and nucleolus) before labeled RNA appeared in the cytoplasm. Examinations of the tritium distribution between uracil and cytosine in RNA of the HeLa tissue culture cell at different times after a limited exposure of the cells to H³-cytidine and continued growth in nonradioactive culture medium supplemented with excess of unlabeled cytidine, demonstrated that the once synthesized RNA was preserved and that the initial distribution of the tritium to the pyrimidine bases was maintained for many hours⁷. This then indicated that the autoradiographically observed shift of labeled RNA indeed represented the movement of macromolecules. Attempts to quantitate the shift of RNA from the nucleus to the cytoplasm in different cell systems have resulted in controversial interpretations as to the role of the chromatin portion of the nucleus and the nucleolus. It was suggested from specific activity time curves in turnover experiments that the chromatin associated RNA is lost while the nucleolar associated RNA contributed to the cytoplasm in liver and pancreas cells⁸.

Supported by the U. S. Atomic Energy Commission. Presented at the International Symposium on Nuclear Energy and Isotopes in Biology and Medicine, Cologne, Germany.