Regulation of rRNA synthesis and processing in animal cells. Effect of nucleoside analogues

The nucleoside analogues fluorouridine and fluorodeoxyuridine (both at 100 muM) and 8-azaguanine (at 500 muM) inhibit both rRNA transcription and processing in Ehrlich ascites cells. In BHK21 cells fluorodeoxyuridine has no effect on either rRNA maturation or transcription, whereas toyocamycin (at 2 microM) inhibits both processes in BHK21 cells and Ehrlich ascites cells. The drugs inhibit transcription in cells incubated in the complete medium, but have no effect on the decreased transcription in cells incubated in a medium without amino acids. This lack of effect cannot be explained by an altered uptake of the drugs in the amino acid-starved cells, since maturation of the rRNA precursor is affected in cells incubated in media with or without amino acids. The effect of the drugs on rRNA transcription is not the consequence of the inhibition of protein synthesis. The results lend support to the proposal that rRNA processing and transcription are co-ordinately controlled in cells with a high rate of rRNA synthesis.