A light-dependent redox signal participates in the regulation of ammonia fixation in chloroplasts of higher plants -: Ferredoxin:Glutamate synthase is a thioredoxin-dependent enzyme

Ferredoxin: glutamate synthase from spinach and soybean chloroplasts has been recognized as a new member of thioredoxin-dependent enzymes in higher plants. For this purpose, a new ferredoxin: glutamate synthase assay procedure had to be established. The assay mixture was filtered through size exclusion filter units to separate the low molecular substrates and products from the proteins thar stop the reaction in the filtrate. This avoids the artificial generation of glutamate by boiling, which was frequently applied to interrupt the reaction by denaturing the enzyme. The glutamate formation was determined by capillary zone electrophoresis. Indications of a thioredoxin-dependency of nitrite reductase and glutamine synthetase have not been observed in combination with different thioredoxin species. Here, we describe for the first time that the fixation of ammonium ions in chloroplasts of higher plants is under thioredoxin control.