Transformation of Aspergillus niger with the homologous nitrate reductase gene

A homologous transformation for Aspergillus niger was developed based on the nitrate reductase structural gene niaD. This system offered certain advantages over existing A. niger systems, such as the ease of recipient mutant isolation, absence of abortive transformants, convenient enzyme assay, ease of transformant stability testing, and complete absence of background growth. Transformation frequencies of up to 100 transformants per microgram DNA were obtained with the vector pSTA10 which carries the niaD gene of A. niger. Southern blotting analysis indicated that vector DNA had integrated into the genome of A. niger. Mitotic stability studies demonstrated that while some transformants were as stable as the wild-type (wt), others were markedly less so. No correlation was seen between plasmid integration, mitotic stability and nitrate reductase activity, which was markedly different from wt in only three of the transformants examined.