Need for DNA topoisomerase activity as a swivel for DNA replication for transcription of ribosomal RNA

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Abstract
Yeast strains with mutations in the genes for DNA topoisomerases I and II have been identified previously in both Saccharomyces cerevisiae1–3 and Schizosaccharomyces pombe4. The topoisomerase II mutants (top2) are conditional-lethal temperature-sensitive (ts) mutants. They are defective in the termination of DNA replication and the segregation of daughter chromosomes2–5, but otherwise appear to replicate and transcribe DNA normally. Topoisomerase I mutants (top1), including strains with null mutations are viable and exhibit no obvious growth defects, demonstrating that DNA topoisomerase I is not essential for viability in yeast1,4,6,7. In contrast to the single mutants, top1 top2 ts double mutants from both Schizosaccharomyces pombe and Saccharomyces cerevisiae grow poorly at the permissive temperature and stop growth rapidly at the non-permissive temperature4,7. Here we report that DNA and ribosomal RNA synthesis are drastically inhibited in an S. cerevisiae top1 top2 ts double mutant at the restrictive temperature, but that the rate of poly(A)+ RNA synthesis is reduced only about threefold and transfer RNA synthesis remains relatively normal. The results suggest that DNA replication and at least ribosomal RNA synthesis require an active topoisomerase, presumably to act as a swivel to relieve torsional stress, and that either topoisomerase can perform the required function (except in termination of DNA replication where topoisomerase II is required).