pch1+, a second essential C-type cyclin gene in Schizosaccharomyces pombe

Abstract

The Schizosaccharomyces pombe gene pch1+ (pombe cyclin C homology) was isolated in a two-hybrid screen for proteins that interact with Cdc2. The cyclin box region of Pch1 protein shares greatest sequence identity with mammalian and Drosophila C-type cyclins (~33% identity). Pch1 is significantly less similar to Mcs2 (19% identity), a second member of the C- type cyclin family in S. pombe. Cdc2 co-precipitates with Pch1 in S. pombe cell lysates, although Cdc2 may not be the major catalytic partner of a Pch1 kinase in vivo. Purified Pch1-associated kinase phosphorylated myelin basic protein, histone H1, and a peptide corresponding to the carboxyl-terminal domain repeat of RNA polymerase II. The amount of pch1 mRNA does not oscillate during the cell cycle, as is the case for mRNA transcripts of other C-type cyclin genes. Δpch1 cells are inviable, therefore S. pombe has two essential genes that encode members of the C-type cyclin family, pch1+ and mcs2+. The Δpch1 mutation causes pleiotropic morphological defects and an associated growth deficiency, but loss of Pch1 activity does not result in a cdc cell cycle-arrest phenotype.