Cloning of the regulatory gene areA mediating nitrogen metabolite repression in Aspergillus nidulans.

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Abstract
The areA gene, which mediates nitrogen metabolite repression in the fungus Aspergillus nidulans, lies sufficiently close to a telomere that no indispensable gene can be distal to it. We were able therefore to exploit the existence of a near terminal pericentric inversion to devise a method for cloning areA plus the region beyond it towards the telomere. In crosses heterozygous for this inversion a class of duplication‐deficient progeny lacking areA and the region centromere‐distal to it is obtained. We, therefore, sought clones from an A. nidulans gene library in lambda Charon 4 able to hybridize to total genomic DNA from a wild‐type strain but not to that from a duplication‐deficiency strain. A clone, containing an 11.6‐kb insert, which hybridised weakly to duplication‐deficiency DNA, overlapped chromosome breakpoints of three different aberration‐associated areA alleles and was able to transform an areA mutant to areA+. Southern blotting and genetic analysis established that the transforming sequence had integrated in the region centromere distal to areA. The cloning method yielded other clones from the region centromere‐distal to areA which were used to show that the translocation associated with a mutant areA allele is reciprocal rather than non‐reciprocal, a fact which could not be established by classical genetics. Finally, analysis of the cloned portion of the dispensable region centromere‐distal to areA indicates that this region contains at least 0.5% of the A. nidulans genome.