Coupling of RNA polymerase III assembly to cell cycle progression in <i>Saccharomyces cerevisiae</i>

Assembly of the RNA polymerases in both yeast and humans is proposed to occur in the cytoplasm prior to their nuclear import. Our previous studies identified a <i>cold-sensitive</i> mutation, <i>rpc128-1007</i>, in the yeast gene encoding the second largest Pol III subunit, Rpc128. <i>rpc128-1007</i> is associated with defective assembly of Pol III complex and, in consequence, decreased level of tRNA synthesis. Here, we show that <i>rpc128-1007</i> mutant cells remain largely unbudded and larger than wild type cells. Flow cytometry revealed that most <i>rpc128-1007</i> mutant cells have G1 DNA content, suggesting that this mutation causes pronounced cell cycle delay in the G1 phase. Increased expression of gene encoding Rbs1, the Pol III assembly/import factor, could counteract G1 arrest observed in the <i>rpc128-1007</i> mutant and restore wild type morphology of mutant cells. Concomitantly, cells lacking Rbs1 show a mild delay in G1 phase exit, indicating that Rbs1 is required for timely cell cycle progression. Using the double <i>rpc128-1007 maf1</i>Δ mutant in which tRNA synthesis is recovered, we confirmed that the Pol III assembly defect associated with <i>rpc128-1007</i> is a primary cause of cell cycle arrest. Together our results indicate that impairment of Pol III complex assembly is coupled to cell cycle inhibition in the G1 phase.