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

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 (similar to 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. Delta pch1 cells are inviable, therefore S. pombe has two essential genes that encode members of the C-type cyclin family, pch1(+) and mcs2(+). The Delta pch1 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.