Evidence for Two Control Genes Regulating Expression of the Quinic Acid Utilization (qut) Gene Cluster in Aspergillus nidulans

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Abstract
The first 3 steps in quinic acid degradation in A. nidulans are catalyzed by highly inducible enzymes encoded by a gene cluster regulated by an adjacent control region. Analysis of 2 non-inducible mutants was done in dipoid strains, where qutA8 is recessive and all 3 enzyme activities were fully induced in heterozygous qutA8/qutA+ diploids. qutA4/qutA+ heterozygous diploids showed semi-dominance of the mutant allele, giving markedly diminished growth on quinic acid and 30-40% decrease of enzyme induction. The qutA4/qutA8 heterozygous diploid grew to the same degree on quinic acid as the qutA4/qutA+ heterozygote and showed the same level of enzyme induction; neither homozygous mutant diploid grew on quiic acid or showed enzyme induction. Therefore the 2 mutant genomes complement, identifying 2 distinct regulatory gene functions. A genetic model is proposed of a negatively acting gene (qutA) repressing expression of a positively acting gene (qutD, previously designated qutA8+) whose product is in turn required for expression of the 3 structural genes. The qutA4 mutation is interpreted to produce an altered repressor insensitive to quinic acid and the qutD8 mutation, the loss of activator protein. Close similarity in the regulation of the quinic acid gene cluster in Neurospora crassa suggests that the 2 types of control mutation, qa1S and qa1F, described for N. crassa may also reflect 2 regulatory genes.