Genetic and molecular analysis of the quinic acid utilization (QUT) gene cluster in Aspergillus nidulans
The first three steps in the utilization of quinic acid in Aspergillus nidulans are catalyzed by highly inducible enzymes encoded by a gene cluster containing three enzyme structural genes and two trans-acting regulatory genes. The gene cluster has been mapped meiotically between the ORNB and FWA genes in linkage group VIII. The regulatory gene QUTD codes for an activator protein, whose function seems to depend upon interaction with the product of the QUTA gene. The QUTA protein thus has a complex role, acting both in repression of gene expression in the absence of the inducer quinic acid and in mediating the role of the QUTD activator protein, upon induction. To test models for the regulation of expression of the QUT enzyme structural genes, the gene cluster has been physically isolated from an A. nidulans genomic DNA library in an X phage replacement vector, using DNA fragments from the equivalent genes in Neurospora crassa as heterologous hybridization probes. An unique 13.2 Kb DNA sequence from A. nidulans was thus isolated and the three structural genes mapped within a 3.5 Kb BamHI fragment. An adjacent region of the cloned DNA shows hybridization to the N. crassa activator gene, QA1F, thus possibly identifying the QUTD gene. No hybridization could be detected with the N. crassa repressor gene, QA1S, which was initially suggested to be equivalent to the QUTA gene of A. nidulans. Evidence against the hypothesis that the QUTA gene encodes a protein with only properties of a repressor is discussed. The QUTE gene which codes for dehydroquinase has been sequenced. Major rearrangements in the coding sequence are found in comparison with the equivalent gene of Neurospora. Analysis of the promoter region of QUTE reveals that it contains a CAAT sequence exhibiting perfect homology with the consensus sequence for eukaryotes, and in close proximity 5' to a sequence showing a dyad of symmetry. The dyad exhibits homology to sequences in other fungal genes identified as binding activator proteins. The importance of these two promoter elements in the control of the expression of the QUTE gene is discussed.