New elements in the mitotic control of the fission yeast Schizosaccharomyces pombe

The p107(wee1) protein kinase plays a central role in regulating the cell cycle of fission yeast. It mediates transmission of signal(s) related to the nutritional status of the cell to the p34(cdc2) protein kinase, which is an active component of the MPF complex driving cells into mitosis. p107(wee1) is itself subject to control by the products of other genes such as nim1+/cdr1+, win1+, and perhaps wis1- and other wis+ genes. At present, the relationships between these genes and their possible roles in the mitotic control are unclear and must await further analysis. It is likely that some of the gene products are concerned with the sensing and/or transmission of nutritional signals. p107(wee1) negatively regulates the activity of p34(cdc2), probably by direct tyrosine phosphorylation, and also appears to regulate the activities of the cdc1+ and cdc27+ gene products. The effects of nitrogen starvation and of wee1 mutations on conditional lethal mutations at the cdc1, cdc2, and cdc27 loci, taken together, support the largely speculative model. During the normal cycle, the balance between phosphorylated and dephosphorylated p34(cdc2) changes such that at the appropriate time, p34(cdc2) is activated and the cell enters mitosis. We suggest that the cdc1+ and cdc27+ products may be regulated in a similar way. Such a mechanism would ensure coordinated activation of these and perhaps other proteins required for the G2/M transition. There are, of course, many uncertainties, and these must await elucidation by biochemical and genetic analysis.