Rapid identification of mutation-complementing restriction fragments from Aspergillus nidulans cosmids

We isolated recombinant DNA cosmid clones that complemented certain mutations in Aspergillus nidulans when covalently closed circular DNA was used to transform cells. We found that DNA from the cosmids was capable of genetic complementation even after digestion to completion with restriction endonucleases that recognize six base pair sequences. Individual restriction fragments were isolated from agarose gels and tested for their ability to complement mutations in cotransformation experiments by using either the A. nidulans trpC or argB gene as selective marker. Specific fragments complemented the mutations. This procedure permits rapid identification of small DNA fragments, from large cosmid inserts, that contain all or part of specific genes, without subcloning the fragments or constructing restriction maps for the cosmids. It therefore significantly simplifies mapping of genes cloned by cosmid complementation or mutations in Aspergillus by providing a starting point for the application of conventional molecular mapping techniques.