Turnover of nuclear and cytoplasmic ribonucleic acid in two types of animal cell, with some further observations on the nucleolus

Abstract

Quantitative radioautographic techniques were used to measure the turnover of ribonucleic acid (RNA) in the nucleus and cytoplasm of 2 animal cells; the connective-tissue cell, which multiplies in vitro, and the macrophage, which does not multiply but remains alive for many hours. Tritium-labelled adenosine and cytidine were used as the precursors and turnover was measured by the rate of release of these precursors from the RNA. In both cells there was a rapid turnover of nuclear RNA. The observed half-life of the nuclear RNA involved in turnover was less than 2 hr. In the macrophage there was also a turnover of cytoplasmic RNA; the observed half-life of this RNA was 5-7 hr. No turnover of cytoplasmic RNA could be detected in the connective-tissue cell over the period studied. The results in both cells appeared to be incompatible with the view that the nuclear RNA was a precursor of the cytoplasmic RNA. In the macrophage the findings indicated that only a small proportion, if any, of the nuclear RNA could have passed into the cytoplasm during the course of the experiments, unless the nuclear RNA''s were rapidly destroyed on entering the cytoplasm. In the connective-tissue cell the magnitude of the counting errors might have permitted more of the nuclear RNA to pass into the cytoplasm without being detected; but, even so, it could be shown that in this cell also most of the nuclear RNA did not pass into the cytoplasm in a stable form. The observed rate of turnover of nucleolar RNA was not significantly different from the observed rate of turnover of the RNA in the rest of the nucleus. There was, however, a higher concentration of RNA in the region of the nucleoli and there was indirect evidence suggesting that the base composition of this RNA was different from that of the RNA in the rest of the nucleus. The chromatin associated with the nucleoli represents a specific part of the chromosomal material which maintains its association with the nucleoli from one cell generation to the next. It is apparently in this chromatin that the reduplication of chromosomal deoxyribonucleic acid is initiated.