title: Characterization of qa-2 mutants of Neurospora crassa by genetic, enzymatic, and immunological techniques.
abstract: Genetic and complementation mapping studies using 20 qa-2 mutants defective for catabolic dehydroquinase indicate that the qa-2 gene encodes a single polypeptide chain and is the structural gene for catabolic dehydroquinase, a 220,000-molecular-weight protein composed of identical 10,000-molecular-weight subunits. Many qa-2 mutants are capable of reversion, but no evidence has yet been obtained for nonsense mutations in this gene. The biochemical consequences of the mutations in two complementing qa-2 strains (M239 and M204) have been determined. Both mutants have extremely low levels of catalytic activity and form a heterocaryon with about 4% of the wild-type activity. As assayed by immunological cross-reactivity, mutant M239 and the heterocaryon have nearly wild-type levels of native-molecular-weight catabolic dehydroquinase protein, whereas M204 has no detectable amount of this protein. Thus it is concluded that M239 has a mutation at or near the catalytic site which reduces the activity 10,000-fold but has little or no influence on the formation of the native multimeric structure. In contrast, M204 apparently has a mutation that severely inhibits aggregation and may have only a minor effect on the inherent potential for catalytic conversion at the reactive site. The heterocaryon would appear to form a mixed multimer with the monomeric subunits from M239 providing the aggregated structure and those from M204, the catalytically active moiety.
mesh_terms: D002874:Chromosome Mapping; D003429:Cross Reactions; D005796:Genes; D005816:Genetic Complementation Test; D006822:Hybrid Cells / Q000201:enzymology; D006836:Hydro-Lyases / Q000096:biosynthesis* / Q000276:immunology / Q000378:metabolism; D005779:Immunodiffusion; D009154:Mutation; D009491:Neurospora / Q000201:enzymology*; D009492:Neurospora crassa / Q000201:enzymology*
chemical_list: D006836:Hydro-Lyases