Noncoordinated transcription in the absence of protein synthesis in yeast

In the yeast Saccharomyces cerevisiae we have carried out a detailed study of the response of transcription to the inhibition of translation. Measurement of the incorporation of labeled bases and nucleosides into the nucleoside triphosphate pools revealed that amino acid deprivation brings about a 10- to 50-fold inhibition of such labeling. Therefore accurate comparisons of RNA synthesis using such precursors are difficult to obtain. To overcome this problem we have turned to the use of L-[methyl-3H] methionine as a precursor, because the labeling of the S-adenosylmethionine pool is relatively unaffected by the rate of protein synthesis. Using this precursor, we have observed that in the absence of protein synthesis the transcription of ribosomal RNA is reduced by 80%, the transcription of messenger RNA is reduced by about 25%, and the transcription of transfer RNA is reduced by less than 20%. These results are obtained when protein synthesis is inhibited either by deprivation of an amino acid or by the addition of cycloheximide. Ribosomal precursor RNA synthesized in the absence of protein synthesis is fully methylated. We conclude that the transcription of rRNA is the primary target of stringent control. Furthermore the inhibition of protein synthesis, itself, may be the trigger for this response.