Abstract
We have cloned a fission yeast gene, cig1 +, encoding a 48 kd product that is most similar to cyclin B proteins. The cig1 + protein has a “cyclin box” ∼40% identical to B-type cyclins of other species, but lacks the “destruction box” required for proteolysis of mitotic cyclins. Deletion of cig1 + had no observable effect on cell viability or progression through G2 or M phase, but instead caused a marked lag in the progression from G1 to S phase. G1 constituted ∼70% of the cell cycle in cig1 deletion strains, as compared with <10% in cit1 + strains. Constitutive cig1 + overexpression was lethal, causing cessation of growth and arrest in G1. Expression of cig1 + failed to rescue an S. cerevisiae strain lacking CLN Start cyclins. Thus, cig1 1 identifies a new class of B-type cyclin acting in G1 or S phase that appears to be functionally distinct from all previously described cyclin proteins.
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