Cell wall assembly ofNeurospora crassa: Isolation and analysis of cell wall-less mutants

Cell-wall-less mutants ofNeurospora crassa were isolated by mutagenizing cells of the temperature-sensitive, protoplast-formingosmotic-1 mutant and screening for cultures that did not regenerate cell wall. Of the 24 strains isolated, 22 were found to have significantly reduced levels of (1–3)-glucan synthase activity; none was found to be defective in chitin synthase activity. Genetic analysis by standard transmission genetics revealed that the glucan synthase defect was ascospore lethal. Complementation analysis by forced heterokaryosis revealed four complementation groups. In each case in which complementation of cell wall formation was observed, there was concomitant restoration of (1–3)-glucan synthase activity. Our results indicate that glucan synthase activity is necessary, but not sufficient, for cell wall assembly and resulting morphogenesis.