The first three steps in quinic acid degradation in are catalysed by highly inducible enzymes encoded by a gene cluster regulated by an adjacent control region. Analysis of two non-inducible mutants has been done in diploid strains, where is recessive and all three enzyme activities are fully induced in heterozygous diploids. In contrast, heterozygous diploids show semi-dominance of the mutant allele, giving markedly diminished growth on quinic acid and 30-40% decrease of enzyme induction. Strikingly, the heterozygous diploid grows to the same degree on quinic acid as the heterozygote and shows the same level of enzyme induction, whereas both the homozygous mutant diploids do not grow on quinic acid and show no enzyme induction. Therefore the two mutant genomes complement, identifying two distinct regulatory gene functions. A genetic model is proposed of a negatively acting gene () repressing expression of a positively acting gene (, previously designated ) whose product is in turn required for expression of the three structural genes. The mutation is interpreted to produce an altered repressor insensitive to quinic acid, and the mutation the loss of activator protein. Close similarity in the regulation of the quinic acid gene cluster in suggests that the two types of control mutation, and qal, described for may also reflect two regulatory genes.


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