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 pomb e is signaled via the spg1p GTPase-switch, which is part of the septation i nitiation network. This is negatively regulated by the two-component GTPase -activating protein (CAP) comprised of the products of the cdc16 and byr4 g enes. Loss-of-function mutations in either of these genes result in multipl e rounds of septum formation without cell cleavage. In this work, we demons trate that attenuation of the protein kinase cdc7p can rescue the lethality of a null allele of cdc16. This observation provides the basis for selecti on of chromosomal mutations and multicopy suppressors that attenuate the si gnaling of septation. Using this screen, mutations in all the previously de scribed septation initiation network genes were obtained, with the exceptio n 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 impli cations for the regulation of septum formation in fission yeast are discuss ed.