Regulation of hypoxanthine transport in Neurospora crassa.

Hypoxanthine uptake and hypoxanthine phosphoribosyltransferase activity (EC 2.4.2.8) were determined in germinated conidia from the adenine auxotrophic strains ad-1 and ad-8 and the double mutant strain ad-1 ad-8. The mutant strain ad-1 appears to lack aminoimidazolecarboximide ribonucleotide formyltransferase (EC 2.1.2.3) or inosine 5'monophosphate cyclohydrolase (EC 3.5.1.10) activities, or both, whereas the ad-8 strain lacks adenylosuccinate synthase activity (EC 6.3.4.4). Normal (or wild-type) hypoxanthine transport capacity was found to the ad-1 conidia, whereas the ad-8 strains failed to take up any hypoxanthine. The double mutant strains showed intermediate transport capacities. Similar results were obtained for hypoxanthine phosphoribosyl-transferase activity assayed in germinated conidia. The ad-1 strain showed greatest activity, the ad-8 strain showed the least activity, and the double mutant strain showed intermediate activity levels. Ion-exchange chromatography of the growth media revealed that in the presence of NH+/4, the ad-8 strain excreted hypoxanthine or inosine, the ad-1 strain did not excrete any purines, and the ad-1 ad-8 double mutant strain excreted uric acid. In the absence of NH+/4, none of the strains excreted any detectable purine compounds.