Induction and Repression of Nitrate Reductase in Neurospora crassa

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Abstract
Synthesis of wild-type N. crassa assimilatory nitrate reductase is induced in the presence of nitrate ions and repressed in the presence of ammonium ions. Effects of several Neurospora mutations on the regulation of this enzyme are shown. The mutants nit-1 and nit-3, involving separate lesions, lack NADPH-nitrate reductase activity and at least 1 of 3 other activities associated with the wild-type enzyme. The 2 mutants do not require the presence of nitrate for induction of their aberrant nitrate reductases and are constitutive for their component nitrate reductase activities in the absence of ammonium ions. An analog of the wild-type enzyme (similar to the nit-1 enzyme) is formed when wild type is grown in a medium in which Mo was replaced by V or W; the resulting enzyme lacks NADPH-nitrate reductase activity. Unlike nit-1, wild type produced this analog only in the presence of nitrate. Contaminating nitrate does not appear to be responsible for the observed mutants'' activities. Nitrate reductase is probably autoregulated. Mutants (am) lacking NADPH-dependent glutamate dehydrogenase activity partially escape ammonium repression of nitrate reductase. The presence of nitrate is required for the enzyme''s induction. A double mutant, nit-1 am-2, proved to be an ideal test system to study the repressive effects of N-containing metabolites on the induction of nitrate reductase activity. The double mutant does not require nitrate for induction of nitrate reductase, and synthesis of the enzyme is not repressed by the presence of high concentrations of ammonium ions. It is repressed by the presence of any one of 5 amino acids. N metabolites (other than ammonium) appear to be responsible for the mediation of ammonium repression.

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