PCT1(-BINDING PARTNER OF P85(CDC10), IS REQUIRED FOR MEIOSIS IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE(), WHICH ENCODES A NEW DNA)

The transcriptional activation of genes at late G(1) is an important r egulatory step in the commitment to a new cell division cycle. In Schi zosaccharomyces pombe, this regulation is mediated by MCB elements tha t serve as binding sites for the MBF/DSC-1 complex. The cdc10(+)-encod ed protein is a component of this complex. We report the cloning of a new gene, pct1(+), encoding a 73-kD protein that interacts with p85(cd c10) to form an MCB-binding heteromer. Pct1(+) is related to, but dist inct from, the res1(+)/sct1(+) gene that also encodes a p85(cdc10) par tner. p73(pct1) has centrally located ankyrin repeats and a putative a mino-terminal DNA-binding domain that has extensive sequence similarit y to the DNA-binding domains of the Saccharomyces cerevisiae SWI4 and MBP1 proteins. The p73(pct1)/p85(cdc10) complex binds both in vitro an d in vivo to MCB but not SCB or E2F sites. Overexpression of pct1(+) i s sufficient to rescue the growth of the cdc10-129 temperature-sensiti ve mutant at the restrictive temperature, although it is unable to res cue a cdc10 null mutation. A deletion of pct1(+) is not lethal but doe s result in a severe meiotic defect. Our results indicate that there a re two cdc10-containing heteromeric complexes that bind to MCB element s and play differential roles in mitotic division and meiosis.