Transfer RNA and ribosomal RNA are synthesized from the same pyrimidine nucleotide pool.

We have tested the hypothesis [Wiegers, Kramer, Klapproth & Hilz (1976) Eur. J. Biochem. 64. 535-540] that the synthesis of rRNA in the nucleolus may have a partially independent supply of nucleoside triphosphates that is not completely suppressed when cells are supplied with exogenous nucleosides. For the slime mould, Physarum polycephalum, and Chinese hamster ovary cells in culture, the specific activities (3H/32P) of UMP and CMP in tRNA, which is synthesised in the nucleoplasm, and rRNA were compared after continuous labelling with [3H]uridine and [32P]phosphate. No differences were found, suggesting that transcription throughout the nucleus draws on a common supply of pyrimidine triphosphates that is uniformly labelled from exogenous [3H]uridine. As a control of the radioactive labelling schedule, a similar experiment was carried out with the prokaryote, Escherichia coli, and identical results were obtained.