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Despite evidence that DNA topoisomerase I is required to relieve torsional stress during DNA replication and transcription, yeast strains with a top1 null mutation are viable and display no gross defects in DNA or RNA synthesis, possibly because other proteins provide overlapping functions. We isolated mutants whose inviability or growth defect is relieved when TOP1 is expressed [trfmutants (topoisomerase one-requiring function)] . The TRF genes define at least four complementation groups. TRF3 is allelic to TOP2. TRF1 is allelic to HPR1, previously shown to be homologous to TOP1 over two short regions. TRF4 encodes a novel 584-amino acid protein with homology to the N-terminus of Saccharomyces cerevisiae topo I. Like top1 mutants, trf4 mutants have elevated rDNA recombination and fail to shut off RNA polymerase II transcription in stationary phase. trf4 null mutants are cs for viability, display reduced expression of GALI and Cell Cycle Box UAS::LacZ fusions, and are inviable in combination with trfI null mutants, indicating that both proteins may share a common function with DNA topoisomerase I. The existence of multiple TRF complementation groups suggests that not all biological functions of topo I can be carried out by topo II.

In Saccharomyces cerevisiae, the processes of meiosis I and meiosis II are regulated by a protein kinase called Cdc28. The active kinase has a catalytic subunit and a regulatory subunit called a cyclin. Nuclear division in eukaryotes may be of three types, meiosis I, meiosis II and mitosis, the differences in the processes lie in the ways in which the sister chromatids are separated. The cell has many cyclins which seem to play specialized roles in meiosis I and meiosis II.