Functional analysis of chitin synthase genes from Aspergillus nidulans

Severe fungal infections are an increasing problem to public health. Most currently available antifungal drugs target the fungal cell membrane, but the fungal cell wall offers the promise of being an additional target for novel drugs. The fungal cell wall has no corresponding structure in humans and the major cell wall polysaccharides, glucans and chitin are not found in humans. The enzyme that catalyzes the reaction that produces chitin, called a chitin synthase, has been explored in detail in Saccharomyces cerevisiae , and to a lesser extent in filamentous fungi. Chitin deposition is spatially and temporally regulated throughout the cell cycle of S. cerevisiae. The vegetative (hyphal) growth and life cycles of a typical filamentous fungus are more complex than that of yeasts. Five chitin synthase enzymes (chsA–E) have been found in the filamentous fungus Aspergillus nidulans, three of which are homologous to chitin synthases in S. cerevisiae, while the other two seem to be unique to filamentous fungi. Previous work has shown that inactivation of the chsB or chsD genes of A. nidulans leads to strains with distinguishable phenotypic defects. We inactivated the chsA, chsC and chsE genes of A. nidulans, and constructed multiple disruptant strains of the chsA, chsB, chsC, chsD, and chsE genes. We found many of the morphological and growth processes in A. nidulans have not only a primary or preferential chitin synthase, but also one or more secondary chitin synthases that serve accessory or backup roles to the primary chitin synthase. The chsA gene is the primary chitin synthase required for conidiophore initiation, but is a secondary chitin synthase in the functions of hyphal and conidiophore development and conidiation. The chsD gene is the primary chitin synthase required for hyphal and conidiophore vesicle wall integrity, but has a secondary role in conidiation and vegetative growth. The chsB gene is primarily required for hyphal morphology and organization. The chsC gene serves a backup role for many developmental processes and the chsE gene appears to have numerous secondary or accessory functions, but neither chsC or chsE appears to have a primary function.