Interaction of cdc2 and rum1 regulates Start and S-phase in fission yeast

The p34(cdc2) kinase is essential for progression past Start in the G(1) phase of the fission yeast cell cycle, and also acts in G(2) to promote mitotic entry. Whilst very little is known about the G(1) function of cdc2, the rum1 gene has recently been shown to encode an important regulator of Start in fission yeast, and a model for rum1 function suggests that it inhibits p34(cdc2) activity. Here we present genetic data suggesting that rum1 maintains p34(cdc2) in pre-Start G(1) form, inhibiting its activity until the cell achieves the critical mass required for Start, and find that in the absence of rum1 p34(cdc2) has increased Start activity in vivo. It is also known that mutation of cdc2, or overexpression of rum1, can disrupt the dependency of S-phase upon mitosis, resulting in an extra round of S-phase in the absence of mitosis. We show that cdc2 and rum1 interact in this process, and describe dominant cdc2 mutants causing multiple rounds of S-phase in the absence of mitosis. We suggest that interaction of rum1 and cdc2 regulates Start, and this interaction is important for the regulation of S-phase within the cell cycle.