Xanthine dehydrogenase expression inNeurospora crassa does not require a functionalnit-2 regulatory gene

Xanthine dehydrogenase (XDH) is the initial enzyme in the purine catabolic pathway ofN. crassa. Secondary nitrogen sources such as purines are metabolized when preferred sources of reduced nitrogen (ammonium or glutamine) are unavailable. XDH synthesis is regulated by glutamine repression and uric acid induction. Thenit-2 locus is believed to encode atrans-acting positive regulator essential for the expression of genes encoding enzymes involved in secondary pathways of nitrogen acquisition, such as XDH and nitrate reductase. However, immunoblot analyses and enzyme assays reveal that XDH protein is synthesized and XDH activity is expressed innit-2 mutants. Nevertheless, XDH responds to nitrogen metabolite repression. The generality thatnit-2 is an obligate control element in nitrogen metabolite repression is questioned. Additionally, mutants defective in XDH activity, namely,xdh-1 and the molybdenum cofactor mutantsnit-1, -7, -8 and -9, are observed to grow on xanthine but not hypoxanthine.This research was supported in part by National Science Foundation Grant DMB 8516203.