THE S-POMBE CDC16 GENE IS REQUIRED BOTH FOR MAINTENANCE OF P34(CDC2) KINASE-ACTIVITY AND REGULATION OF SEPTUM FORMATION - A LINK BETWEEN MITOSIS AND CYTOKINESIS

In the fission yeast Schizosaccharomyces pombe, septum formation and cytokinesis are dependent upon the initiation, though not the completion of mitosis. A number of cell cycle mutants which show phenotypes consistent with a defect in the regulation of septum formation have been isolated. A mutation in the S.pombe cdc16 gene leads to the formation of multiple septa without cytokinesis, suggesting that the normal mechanisms that limit the cell to the formation of a single septum in each cycle do not operate. Mutations in the S.pombe early septation mutants cdc7, cdc11, cdc14 and cdc15 lead to the formation of elongated, multinucleate cells, as a result of S phase and mitosis continuing in the absence of cytokinesis. This suggests that in these cells, the normal mechanisms which initiate cytokinesis are defective and that they are unable to respond to this by preventing further nuclear cycles. Genetic analysis has implied that- the products of some of these genes may interact with that of the cdc16 gene. To understand how the processes of septation and cytokinesis are regulated and coordinated with mitosis we are studying the early septation mutants and cdc16. In this paper, we present the cloning and analysis of the cdc16 gene. Deletion of the gene shows that it is essential for cell proliferation: spores lacking a functional cdc16 gene germinate, complete mitosis and form multiple septa without undergoing cell cleavage. Examination of the activity of p34cdc2 kinase in the cdc16-116 mutant shows that, after shift to the non-permissive temperature, mitotic p34cdc2 kinase activity decays rapidly and cells fail to respond normally to the absence of the mitotic spindle, initiating septum formation even if mitosis has not been completed. The sequence of the predicted gene product shows homology to the BUB2 gene of Saccharomyces cerevisiae. Expression of the BUB2 gene in S.pombe will rescue the cdc16-116 mutant, but not a null allele. The cdc16 gene product may be part of a feedback mechanism preventing premature exist from mitosis by maintaining high levels of p34cdc2 activity until the initiation of anaphase, thereby coordinating mitosis with septation, cytokinesis and entry into the next cycle.