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Specht, C. A., Liu, Y., Robbins, P. W., Bulawa, C. E., Iartchouk, N., Winter, K. R., Riggle, P. J., Rhodes, J. C., Dodge, C. L., Culp, D. W., and Borgia, P. T. 1996. ThechsDandchsEgenes ofAspergillus nidulansand their roles in chitin synthesis.Fungal Genetics and Biology20,153–167. Two chitin synthase genes,chsDandchsE,were identified from the filamentous ascomyceteAspergillus nidulans.In a region that is conserved among chitin synthases, the deduced amino acid sequences ofchsDandchsEhave greater sequence identity to the polypeptides encoded by theSaccharomyces cerevisiae CHS3gene (also namedCSD2, CAL1, DIT101,andKTI1) and theCandida albicans CHS3gene than to other chitin synthases.chsEis more closely related to theCHS3genes, and this group constitutes the class IV chitin synthases.chsDdiffers sufficiently from the other classes of fungal chitin synthase genes to constitute a new class, class V. Each of the wild-typeA. nidulansgenes was replaced by a copy that had a substantial fraction of its coding region replaced by theA. nidulans argBgene. Hyphae from bothchsDandchsEdisruptants contain about 60–70% of the chitin content of wild-type hyphae. The morphology and development ofchsEdisruptants are indistinguishable from those of wild type. Nearly all of the conidia ofchsDdisruption strains swell excessively and lyse when germinated in low osmotic strength medium. Conidia that do not lyse produce hyphae that initially have normal morphology but subsequently lyse at subapical locations and show ballooned walls along their length. The lysis of germinating conidia and hyphae ofchsDdisruptants is prevented by the presence of osmotic stabilizers in the medium. Conidiophore vesicles fromchsDdisruption strains frequently swell excessively and lyse, resulting in colonies that show reduced conidiation. These properties indicate that chitin synthesized by thechsD-encoded isozyme contributes to the rigidity of the walls of germinating conidia, of the subapical region of hyphae, and of conidiophore vesicles, but is not necessary for normal morphology of these cells. The phenotypes ofchsDandchsEdisruptants indicate that the chitin synthesized by each isozyme serves a distinct function. The propensity of achsDdisruptant for osmotically induced lysis was compared to that of strains carrying two other mutations (tsE6andorlA::trpC) which also result in reduced chitin content and vegetative cell lysis. The concentration of osmotic stabilizer necessary to remedy the lysis of strains carrying the three mutations is inversely related to the chitin content of each strain. This finding directly demonstrates the importance of chitin to the integrity of the cell wall and indicates that agents that inhibit thechsD-encoded chitin synthase could be useful anti-Aspergillusdrugs. |
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