Cytoplasmic and nuclear pyrimidine ribonucleotide pools in HeLa cells

A comparison of the rate of approach to equilibrium U:C labeling for two species of RNA whose base composition is known has been described previously as a method to determine the similarity or difference in the pyrimidine ribonucleotide pools from which the RNA species are synthesized. The procedure is now utilized to examine the pools used for the synthesis of HeLa cell nuclear RNA, mitochondrial RNA and poliovirus RNA. The results indicate that poliovirus RNA molecules derive from a pyrimidine precursor pool indistinguishable from that used for the synthesis of both heterogeneous nuclear RNA and 45 S ribosomal RNA in the nucleus, but that a separate and distinct pool is used for the synthesis of mitochondrial RNA. In addition, a comparison of the radioactivity ultimately found in UMP and CMP in a single species of RNA labeled in the presence or absence of actinomycin D shows that the conversion of uridine to cytidine nucleotides is significantly slowed in the presence of the drug. Direct measurement of the ratios of radioactivity in these RNA precursors in the acid-soluble pool confirm this observation.