Isolation and characterization of pos mutants defective in correct positioning of septum in Schizosaccharomyces pombe.

We have isolated mutants of fission yeast defective in correct positioning of septum. In visual screening, we obtained 16 clones showing unequal septation at restrictive temperature, which were classified into three complementation groups. At restrictive temperature, all the mutants underwent nuclear division normally. In cytokinesis, however, a contractile ring was formed at the site independent of the mitotic spindle. These results suggest that positional information for cytokinesis are not accurately transmitted to the cell equator. Furthermore, all the mutants frequently displayed incorrect orientation and/or distortion of septum, which suggests that the septum positioning is closely related to correct orientation and organization of septum.