The chsB gene of Aspergillus nidulans is necessary for normal hyphal growth and development

The chsB gene from Aspergillus nidulans encodes a class III chitin synthase, an enzyme class found in filamentous fungi but not in yeast-like organisms. Using a novel method, we isolated haploid segregants carrying a disrupted chsB allele from heterozygous diploid disruptants. The haploid disruptants grow as minute colonies that do not conidiate. Hyphae from the disruptants have enlarged tips, a high degree of branching, and disorganized lateral walls. The mycelium is not deficient in chitin content and shows no evidence of lysis. The disruptant phenotype is not remedied by osmotic stabilizers. The results indicate that chitin synthesized by the chsB-encoded enzyme does not substantially contribute to the rigidity of the cell wall but is necessary for normal hyphal growth and organization. The properties of the A. nidulans disruptant are similar to those for Neurospora crassa strains with a disrupted chs-1 gene, which also encodes a class III chitin synthase. The morphology of an A. nidulans heterokaryon containing both the wild-type and the disrupted chsB alleles indicates that chsB acts in local areas of the mycelium. The heterokaryon produces conidia of both parental genotypes in nearly equal numbers, indicating that the wild-type chsB gene is not necessary for conidium formation. In addition, we identified and sequenced a second, previously undescribed, homolog of chsB from the closely related opportunistic pathogen, A. fumigatus. The finding of two class III chitin synthase genes in A. fumigatus and a single gene of this class in A. nidulans illustrates limitations of using A. nidulans as a genetic model for A. fumigatus.