The induction and repression of nitrate reductase in the fungus Aspergillus nidulans.

Abstract

Summary The enzyme nitrate reductase (NADPH: nitrate oxidoreductase, EC 1.6.6.3) is present only in mycelium grown in the presence of nitrate and absence of ammonium. Techniques have been devised for studying the kinetics of nitrate reductase appearance in mycelium upon the addition of nitrate to the medium. Changes in the level of nitrate reductase in mycelium transferred from nitrate to nitrate, ammonium, and glutamate, used singly or in combination as nitrogen sources have also been studied. The result of such experiments indicate that the synthesis of nitrate reductase is induced by nitrate and repressed by ammonium. No effect in vitro of ammonium could be found. Upon induction at 25° there was a lag of about 8 min before enzyme began to appear. The rate of production per unit volume of medium thereafter decreased for about 1 h, and then became constant, even though mycelial mass per unit volume of medium increased throughout the experiment. At no time therefore, in contrast to most Escherichia coli systems, was rate of induction directly proportional to mycelial mass.