Control of timing of cell cycle events in fission yeast by the wee 1⁺ gene

Abstract

The means by which cells control the timing of events in the cell division cycle have been investigated in depth recently. The existence of controls over the major events of DNA replication and mitosis has been demonstrated in several systems; controls of both events have been identified in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The control points have been defined on the basis of cell cycle arrest caused by certain mutations^1–4 or mating pheromones⁵, and by growth-related parameters^3,6–10. Investigation of how the component processes, generally identified by specific genes, are organized into developmental pathways^11,12 has provided much information on how the proper temporal order of events is maintained, but little is known about the control of the absolute timing of the component processes in the cycle. The time of completion of a gene-controlled process in the cell cycle can be estimated from the transition or execution point of a temperature-sensitive mutant defective in the particular gene¹³. In the fission yeast S. pombe the transition points of most mutants defective for mitosis are temporally clustered shortly before the event¹⁴, but the mechanism which maintains this coordination has not been investigated. I report here that the transition points of three genes required for mitosis are advanced in strains carrying a wee 1 mutation, whose best known effect is to reduce cell size at mitosis¹⁵. The observed advancement can be as much as 0.4 of the cycle, implicating the wee 1⁺ gene in a system which controls the timing and temporal coordination of developmental steps controlled by three mitotic genes.