Induction and synthesis of nitrate reductase in Chlorella vulgaris

Abstract

Assimilatory nitrate reductase is an inducible, eukaryotic enzyme that responds to a variety of environmental cues. When higher plants and green algae are grown with ammonia as a nitrogen source, low levels of nitrate reductase activity are present. Transfer to nitrate-containing medium is accompanied by substantial increase of nitrate reductase activity. Here it is shown immunologically that, in the green algae Chlorella vulgaris, nitrate reductase protein is over-produced as activity appears during induction. Immunoreactive protein is also found in cells grown on ammonia. Low levels of translatable mRNA for nitrate reductase are present in ammonia-grown cells. These data suggest that: (i) nitrate reductase appearance is controlled primarily on a transcriptional level, but that transcription is not completely halted under repressing conditions; (ii) there is an overproduction of nitrate reductase protein early during the induction period as previously suggested; and (iii) nascent protein, from in vitro translation, is of approximately the same molecular size as the nitrate reductase subunit and therefore little post-translational modification is necessary to generate the functional enzyme. Insertion of cofactors and assembly are probably the only post-translational events.