In vivo and in vitro studies of nitrate reductase regulation inAspergillus nidulans

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Abstract
Induced wildtype cells ofA. nidulans rapidly lost NADPH — linked nitrate reductase activity when subjected to carbon and or nitrogen starvation. A constitutive mutant at the regulatory gene for nitrate reductase,nirA c1, rapidly lost nitrate reductase activity upon carbon starvation. This loss of activity is thought to be due to a decrease in the NADPH concentration in the cells. Cell free extracts from wild-type cells grown in the presence of nitrate, rapidly lost their nitrate reductase activity when incubated at 25° C. NADPH prevented this loss of activity. Wildtype cells grown in the presence of nitrate and urea have a higher initial NADPH: NADP+ ratio and cell free extracts from such cells lost their nitrate reductase activity slower than extracts of cells grown with nitrate alone. The Pentose Phosphate Pathway mutant,pppB-1, had a lower NADPH concentration compared with the wildtype grown under the same conditions and cell free extracts lost their nitrate reductase activity more rapidly than the wildtype. Cell free extracts ofnirA c-1 and a non-inducible mutant for nitrate reductase,nirA --14, upon incubation lost little of their nitrate reductase activity.

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