Studies on the regulation of RNA synthesis in the yeast, Saccharomyces cerevisiae.

The stringent control of RNA synthesis in the yeast Saccaromyces cerevisiae may be evoked either by starving for a required amino acid or by inhibiting protein synthesis. The response is non-coordinate in that the synthesis of ribosomal and messenger RNA is depressed whereas that of transfer RNA continues. If protein synthesis is blocked in starved cells then tRNA synthesis is stimulated.

In this thesis the relationship between the level of tRNA charging and the transcriptional and translational state of the yeast cell has been examined. When the cells are starved for an amino acid the corresponding tRNA species only becomes uncharged. This effect can be counteracted by the addition of protein synthesis inhibitors to the starved cells. In contrast, the same inhibitors provoked the discharge of tRNA in growing (non-starved) yeast. Similar results were obtained when protein synthesis was blocked using a temperature-sensitive mutant. These contrasting effects of translation inhibition on tRNA charging in starved and non-starved cells correlate with the changes that inhibition evoked in the transcriptional state of those cells. The data appear to indicate that tRNA synthesis is under autoregulatory control and that tRNA charging may also play an important role in the regulation of rRNA synthesis.

A study has also been made of the regulation of the synthesis of P1 doublestranded (ds) RNA, the genome of the yeast virus-like particle (VLP). When protein synthesis is prevented by starvation for a required amino acid or by addition of cycloheximide, the rate of P1 dsRNA synthesis is reduced markedly. During nitrogen starvation the synthesis of P1 dsRNA persists but is accompanied by the degradation of pre-existing molecules. This degradation appears to require the induction of new enzymes and it is likely that the breakdown products are used to enable the cell to complete its division cycle. However, not all of the copies of the VLP genome are degraded in this process, some are conserved and can replenish the amount of P1 dsRNA on return to growth conditions. These results are discussed with respect to possible models for dsRNA replication and the controls which appear to operate on this process are compared to those exerted on nuclear RNA synthesis.