Borgia P T, Iartchouk N, Riggle P J, Winter K R, Koltin Y, Bulawa C E
Department of Medical Microbiology and Immunology, Southern Illinois University School of Medicine, Springfield 62794-9230, USA.
Fungal Genet Biol. 1996 Sep;20(3):193-203. doi: 10.1006/fgbi.1996.0035.
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.
构巢曲霉的chsB基因编码一种III类几丁质合酶，这是一种在丝状真菌中发现但在酵母样生物体中未发现的酶类。我们采用一种新方法，从杂合二倍体破坏株中分离出携带chsB等位基因破坏的单倍体分离株。单倍体破坏株长成微小菌落，不产生分生孢子。破坏株的菌丝顶端膨大、分支程度高且侧壁无序。菌丝体几丁质含量不缺乏且无裂解迹象。破坏株表型不能通过渗透稳定剂得到补救。结果表明，由chsB编码的酶合成的几丁质对细胞壁的刚性没有实质性贡献，但对正常的菌丝生长和组织是必需的。构巢曲霉破坏株的特性与chs - 1基因被破坏的粗糙脉孢菌菌株相似，chs - 1基因也编码一种III类几丁质合酶。含有野生型和破坏的chsB等位基因的构巢曲霉异核体的形态表明chsB在菌丝体的局部区域起作用。异核体产生几乎数量相等的两种亲本基因型的分生孢子，表明野生型chsB基因对分生孢子形成不是必需的。此外，我们从密切相关的机会致病菌烟曲霉中鉴定并测序了chsB的第二个此前未描述的同源物。在烟曲霉中发现两个III类几丁质合酶基因，而在构巢曲霉中只有一个此类基因，这说明了将构巢曲霉用作烟曲霉遗传模型的局限性。
