Changes in the rate of ribosomal RNA synthesis during the cell cycle in Saccharomyces cerevisiae

Abstract

In Saccharomyces cerevisiae DNA is synthesized discontinuously during the cell cycle. To determine further the effect on RNA synthesis of the doubling of the ribosomal gene cistrons, the rate of ribosomal RNA (rRNA) synthesis at different stages of the cell cycle was examined. Exponential cultures of 14C-adenine labelled cells of S. cerevisiae were labelled with 32P-phosphate for 10 min. The cells were chilled, fractionated on a zonal rotor according to cell age, the RNA extracted, different RNA species isolated by SDS polyacrylamide gel electrophoresis, and counted for 14C and 32P. 32P incorporation was lowest in the early stages of the cell cycle and increased with cell age. Labelled RNA precursors (35S, 27S, and 20S) were readily detected in the early stages of the cell cycle. These results suggest that transcription and processing of ribosomal RNA varies throughout the cell cycle and is not directly related to gene dosage. An apparent increase in the rate of rRNA synthesis (as measured by isotope incorporation) could be a result of either a true increase or an increase in the specific activities of the immediate precursors of RNA synthesis. Measurements of the XTP intracellular pools indicated that the specific activities of these RNA precursors did not change during the cell cycle.