Septum pattern in ts mutants of <i>Schizosaccharomyces pombe</i> defective in genes <i>CDC3, CDC4, CDC8</i> and <i>CDC12</i>

Septum-defective mutants of Schizosaccharomyces pombe impaired in cdc genes 3, 4, 8 and 12 were compared by fluorescence microscopy, freeze-etching and ultrathin sectioning. This approach made it possible to recognize the internal organization of defective phenotypes under restrictive conditions. Of special interest in this study was the pattern of unusual septum malformations found to be regular features of the terminal phenotypes of the mutants. Their overall topology was visualized at the cellular level by primulin fluorescence. The subcellular location of septum defects was found to be identical in origin to the compartment where normal septum was assembled in the wild type. Delocalized septation involved both microfibrillar and matrix components, which participated in the final assembly of malformations. Unique contour views of delocalized septa were exposed by freeze-fracturing. Cytoplasmic microtubules and microfilaments were detected in ultrathin sections of the cytoplasm of mutant cells. The internal organization of malformation-accumulating phenotypes suggested a disruption of the directional mechanism that steers septum material to the periplasm at the cell equator.