Simultaneous induction and repression of nitrate reductase and TPNH cytochrome c reductase in Neurospora crassa

1.Amino acid, pyrimidine, and purine-requiring mutants of Neurospora all require their respective supplements for the induction of TPNH cytochrome c reductase (TPNH: cytochrome c oxidoreductase, EC 1.6.2.3) and nitrate reductase (TPNH : nitrate oxidoreductase, EC 1.6.6.3). The level of this requirement in at least one case was the same as that for growth. Growth inhibitors inhibited the induction of the two activities in question.2.TPNH cytochrome c reductase and nitrate reductase were both induced by nitrate, but isocitrate lyase (L8-isocitrate glyoxalate-lyase, formerly known as iso-citratase, EC 4.1.3.1) was not. 2-Methylalanine repressed the above reductases, but had no effect on the induction of isocitrate lyase. 2-Methylalanine did not inhibit either of the reductases in question in vitro.3.The ratio of the activity of TPNH cytochrome c reductase to nitrate reductase remained reasonably constant throughout a 90% heat inactivation at 42.3° and during a greater than 50% heat inactivation at 31.1°.4.The following hypothesis is proposed to account for the above and for previously made observations : nitrate reductase is an aggregate of two polypeptides. One transports electrons from TPNH to FAD and from there to cytochrome c and another accepts electrons from the reduced FAD in the first polypeptide, passes them to molybdate, and from there to nitrate.