Enhanced labelling of RNA synthesized in nuclei and reduced labelling of RNA synthesized in the cytoplasm by [32P]orthophosphate in the presence of nucleosides.

Labelling of cellular RNA by [32P]orthophosphate can be enhanced up to a factor of three by adding 2 to 10 X 10(-5)M of non-labelled nucleosides to the culture medium. The lag period of labelling can be reduced by a factor of two. Adenosine has the highest effect, while uridine is without effect. Labelling of viral RNA, which occurs in the cytoplasm, is reduced by nucleosides, suggesting that two precursor pools of nucleoside triphosphates are differently affected. The base composition of the labelled RNA cannot be influenced by incubation with non-labelled nucleosides. As a possible explanation for this observation a group translocation transport mechanism is discussed.