An examination of the ribonucleic acids in the HeLa cell with special reference to current theory about the transfer of information from nucleus to cytoplasm

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Abstract
The experiments described in the present paper were carried out on HeLa cells growing exponentially in suspension culture. The nuclei were isolated by the procedure of Fisher & Harris (1962) which permits the recovery of virtually all the nuclei from a population of cells. These nuclei contained between 98 and 99% of the deoxyribonucleic acid (DNA) of the cells and, within the limits of measurement, all the ribonucleic acid (RNA) in the nucleus labelled in a 10 min exposure of the cells to a radioactive RNA precursor. The residual cytoplasm was separated by centrifugation into a microsomal and a supernatant fraction. The ribonucleic acids extracted from the nuclei and from the two cytoplasmic fractions were examined in the ultracentrifuge. If care was taken to minimize degradation, the rapidly labelled nuclear RNA sedimented as two components having sedimentation coefficients of approximately 28S and 16S. The microsomal RNA, when prepared in the same way, also had two components, with sedimentation coefficients similar to those of the nuclear RNA. The cytoplasmic supernatant fraction contained, apart from a major component sedimenting at approximately 3S, two minor components sedimenting at 28S and 16S. These minor components were not derived from the rapidly labelled nuclear RNA, but from the microsomal RNA. The passage of radioactivity through these ribonucleic acids was studied in experiments in which the cells were first exposed for 10 min to a radioactive RNA precursor and then incubated for 5 h in non-radioactive medium. The results indicated that the nuclear RNA which became labelled rapidly, and which had been shown in previous experiments to be broken down within the cell, was not transferred to the microsomes in the cytoplasm before being broken down. The observations suggested that this labile nuclear RNA was broken down within the nucleus. This interpretation of the data was strengthened by the finding that the nucleus, but not the cytoplasm, contained an active enzyme system which degraded the rapidly labelled nuclear RNA to nucleoside-5$^{\prime}$-monophosphates; there is reason to believe that this RNA is also broken down to nucleoside-5$^{\prime}$-monophosphates in the intact cell. The base composition of the nuclear and cytoplasmic ribonucleic acids was examined by conventional chemical methods and also by studying the distribution of radioactivity in the RNA nucleotides after the cells had been exposed for short periods to radioactive phosphorus. No RNA was revealed which had a base composition similar to that of the DNA. The relevance of these findings to current theory about the mechanism by which information is transferred from nucleus to cytoplasm is discussed.