Genetic Analysis of the First Steps of Sulphate Metabolism in Aspergillus nidulans

Abstract

PHENOTYPIC analysis of interactions between different mutations in the same genome often helps elucidate the structure and function of the loci in question. If, in a series of mutations affecting a linear pathway, each non-allelic mutation has a characteristic phenotype, then the order of the steps they affect can be determined from the phenotypes of double mutants. A (“non-leaky”) mutation blocking one step in the pathway will be epistatic to all those blocking later steps. The minimum number of different double mutants sufficient to determine unequivocally the order of loci participation on a linear pathway equals the number of different loci (the total number of different double mutants equals n(n−l)/2 where n equals the number of different loci). This communication reports application of this method in Aspergillus nidulans to four loci where mutation can result in loss of ability to use sulphate but not sulphite as a sulphur source and indicates that these four loci are involved in the conversion of sulphate to a metabolite which interferes with utilization of exogenous choline O-sulphate.