A role for the START gene-specific transcription factor complex in the inactivation of cyclin B and Cut2 destruction

Hyperactivation of Cdc2 in fission yeast causes cells to undergo a lethal p remature mitosis called mitotic catastrophe. This phenotype is observed in cdc2-3w wee1-50 cells at high temperature. Eleven of 17 mutants that suppre ss this phenotype define a single complementation group, mcs1. The mcs1-77 mutant also suppresses lethal inactivation of the Weel and Mik1 tyrosine ki nases and thus delays mitosis independently of Cdc2 tyrosine phosphorylatio n. We have cloned mcs1 by isolating suppressors of the cell cycle arrest ph enotype of mcs1-77 cdc25-22 cells and found that it encodes Res2, a compone nt of the START gene-specific transcription factor complex MBF (also known as DSC-1). The mcs1-77 mutant bears a single point mutation in the DNA-bind ing domain of Res2 that causes glycine 68 to be replaced by a serine residu e. Importantly, two substrates of the anaphase-promoting complex (APC), the major B-type cyclin, Cdc13, and the anaphase inhibitor, Cut2, are unstable in G2-phase mcs1-77 cells. Consistent with this, we observe abnormal siste r chromatid separation in mcs1-77 cdc25-22 cells at the restrictive tempera ture. Mutation of either Cdc10 or Res1 also deregulates MBF-dependent trans cription and causes a G2 delay. We find that this cell cycle delay is aboli shed in the absence of the APC regulator Ste9/Srw1 and that the periodic ex pression of Ste9/Srw1 is controlled by the MBF complex. These data suggest that in fission yeast the MBF complex plays a key role in the inactivation of cyclin B and Cut2 destruction by controlling the periodic production of APC regulators.