Deficiency in both type I and type II DNA topoisomerase activities differentially affect rRNA and ribosomal protein synthesis in Schizosaccharomyces pombe

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Abstract
The synthesis of rRNA and r-proteins was studied in temperature-sensitive topoisomerase mutants of the fisson yeast Schizosaccharomyces pombe. To reduce the severity of heatshock response seen in the wild type strain, slow temperature shift-up of the cultures was used to inactivate the mutant topoisomerases. It was found that the temperature shift caused a large preferential reduction of rRNA synthesis in the top1top2 double mutant. In contrast, no preferential inhibition of rRNA synthesis was observed in top1 or top2 single mutants, although some reduction in the total RNA synthesis was observed in the top2 mutant. Thus, as observed with Saccharomyces cerevisiae (Brill et al. 1987), relaxation of supercoiled DNA structures by either topoisomerase I or II appears to be essential for efficient transcription of rRNA genes. Analysis of r-protein synthesis indicated that there were small decreases in the differential synthesis rates of r-proteins after temperature shift-up in the top1top2 mutant, but the observed negative effects on r-protein synthesis was much smaller than that on rRNA synthesis, and degradation of the newly synthesized r-proteins was observed. These observations indicate the apparent lack of tight coupling between rRNA and r-protein synthesis in S. pombe under these experimental conditions.