THE SCHIZOSACCHAROMYCES-POMBE CDC5(+) GENE ENCODES AN ESSENTIAL PROTEIN WITH HOMOLOGY TO C-MYB

The Schizosaccharomyces pombe cdc5(+) gene was identified in the first screen for cell division cycle mutants in this yeast. The cdc5(+) gene was reported to be required for nuclear division but because of its modest elongation and leaky nature at the non-permissive temperature, it was not investigated further. Here, we report the characterization of the single allele of this gene, cdc5-120, in more detail. The mutant arrests with a 2N DNA content and a single interphase nucleus. Further genetic analyses suggest that cdc5(+) gene function is essential in the G(2) phase of the cell cycle. We have cloned and sequenced the cdc5(+) gene. The deduced protein sequence predicts that Cdc5 is an 87 kDa protein and contains a region sharing significant homology with the DNA binding domain of the Myb family of transcription factors. Deletion mapping of the cdc5(+) gene has shown that the N-terminal 232 amino acids of the protein, which contain the Myb-related region, are sufficient to complement the cdc5(ts) strain. A cdc5 null mutant was generated by homologous recombination. Haploid cells lacking cdc5(+) are inviable, indicating that cdc5(+) is an essential gene. A fusion protein consisting of bacterial glutathione S-transferase joined in-frame to the N-terminal 127 amino acids of the Cdc5 protein is able to bind to DNA cellulose at low salt concentrations. This evidence suggests that cdc5(+) might encode a transcription factor whose activity is required for cell cycle progression and growth during G(2).