POSITIVE REGULATION IN A EUKARYOTE, A STUDY OF THE uaY GENE OF ASPERGILLUS NIDULANS: I. CHARACTERIZATION OF ALLELES, DOMINANCE AND COMPLEMENTATION STUDIES, AND A FINE STRUCTURE MAP OF THE uaY - oxpA CLUSTER

摘要

In this paper we characterize genetically a positive eukaryotic regulatory gene: the ua Y gene of the ascomycete Aspergillus nidulans . Several steps in the uptake and degradation of purines are under the control of the ua Y gene (summarized in Scazzocchio and Gorton 1977). In the present paper 12 ua Y- mutations are characterized with respect to their inducibility for adenine deaminase, xanthine dehydrogenase (purine hydroxylase I) and urate oxidase and by the absence of the uric acid-xanthine permease scored in vivo by resistance to 2-thiouric acid. While 10 mutations are uniformly unleaky, two others are almost wild type for the induction of urate oxidase. A fine structure map of the ua Y gene shows that the two "leaky" mutations are not clustered. The fine structure mapping unambiguously positions six ua Y alleles and provides preliminary but interesting trends regarding the pattern of gene conversion in the ua Y gene. The enzyme levels in all ua Y-/ ua Y+ heterozygous diploids are intermediate between the corresponding ua Y-/ ua Y- and ua Y+/ ua Y+ homozygous diploids, suggesting that one functional copy of the ua Y gene is able to mediate the complete induction of only one set of structural genes. No complementation was found between any two ua Y- alleles. This establishes that the mutations showing either of the phenotypes are alleles in the same gene; it fails to provide evidence for intracistronic complementation. A mutation, oxp A5, causes resistance to the xanthine analogue oxypurinol (4, 6-dihydroxypyrazolo-(3, 4-d)-pyrimidine) and partial constitutivity of adenine deaminase, xanthine dehydrogenase (purine hydroxylase I) and urate oxidase. The constitutive phenotype is suppressed by mutations blocking the synthesis of intracellular inducers. The mutation is recessive and complements fully with the 11 ua Y- mutations tested. It maps to the left of all 12 ua Y mutations to which it has been crossed. The data indicate that both the resistance and constitutivity arise from one mutational event in a gene, oxp A, different from ua Y and possibly adjacent to it. We propose that the oxp A gene codes for a protein involved in limiting the flow of inducers into the cell nucleus. Thus oxp A and ua Y constitute a regulatory gene cluster, indicating that ua Y is the regulatory gene.