Bypass of the requirement for cdc16p GAP function in Schizosaccharomyces pombe by mutation of the septation initiation network genes

The onset of septum formation in the fission yeast Schizosaccharomyces pombe is signaled via the spg1p GTPase-switch, which is part of the septation initiation network. This is negatively regulated by the two-component GTPase-activating protein (CAP) comprised of the products of the cdc16 and byr4 genes. Loss-of-function mutations in either of these genes result in multiple rounds of septum formation without cell cleavage. In this work, we demonstrate that attenuation of the protein kinase cdc7p can rescue the lethality of a null allele of cdc16. This observation provides the basis for selection of chromosomal mutations and multicopy suppressors that attenuate the signaling of septation. Using this screen, mutations in all the previously described septation initiation network genes were obtained, with the exception of byr4, sid4 and plo1. We also demonstrate that increased expression of the dma1 gene can rescue the lethality of a null allele of cdc16. The implications for the regulation of septum formation in fission yeast are discussed.