THE CELL-CYCLE GENES CDC22-POMBE ENCODE THE LARGE AND SMALL SUBUNITS OF RIBONUCLEOTIDE REDUCTASE( AND SUC22+ OF THE FISSION YEAST SCHIZOSACCHAROMYCES)

The cdc22+ gene of Schizosaccharomyces pombe is required early in the cell cycle, and its transcript varies in concentration in step with th e cell cycle, with a peak level at the G1-S boundary. The sequences of the cdc22+ gene and of a multicopy suppressor of cdc22ts mutations, s uc22+, have been determined. The cdc22+ open reading frame, which is i nterrupted in the genome by a single intron very close to its 5' end, encodes a protein of 811 amino acids, which has an amino acid sequence highly similar to that of the large subunit of ribonucleotide reducta se from several species. The suc22+ gene contains an uninterrupted ope n reading frame of 391 amino acids, very similar to the sequence of th e small subunit of ribonucleotide reductase. Disruption of either gene is lethal. Upstream of the cdc22+ coding region are seven short seque nce elements similar to the recognition sequence for MluI, which are i nvolved in regulating periodic transcription of the gene. Inhibition o f DNA synthesis by hydroxyurea results in a several-fold increase in t he level of the cdc22+ transcript. In contrast, hydroxyurea does not i nduce the 1.5 kb transcript of suc22+, but results in the induction of a 1.9 kb mRNA which hybridises to suc22+ DNA.