Morphological mutants represent roughly 23% of seven hundred-odd distinct chromosomal loci of N. crassa, as listed by Perkins et al. (1982, Microbiol. Rev. 46:426). Probably the most radical phenotype among these strains is that of the fz;sg;os-1 ("slime") triple mutant, which was isolated by Sterling Emerson (1963, Genetica 34:162) in a mutagenic experiment using an os-1 strain. The "slime" strain has been systematically referred to in the literature as "a strain lacking cell wall and growing as protoplasts or plasmodium" (Perkins et al. 1982). Through the years, the fragile "slime" structures were frequently used as a source of organelles (Martinoia et al. 1979. Arch. Microbiol. 120:31), membranes (Scarborough, 1975. J. Biol. Chem. 250:1106) or for the study of membrane-bound enzymes (Brooks et al. 1983. J. Biol. Chem. 258:13909). "Slime" spheroplasts practically never revert to hyphal morphology; thus, the causes for impaired cell wall synthesis were investigated and attributed either to the lack of glucan synthase activity (Leal-Morales and Ruiz-Herrera, 1985. Exp. Mycol 9:28) or to improper ultrastructural characteristics of the organelles responsible for chitin synthesis: the chitosomes (Martinez et al. 1989. Biochem. Biophys. Acta 990:45).

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