In the green alga Chlamydomonas reinhardtii, NR is an enzyme of 220 kDa consisting of 2 kinds of subunits each responsible for the partial activities NR-diaphorase and BVH-NR.

NR is regulated by an interconversion process centered upon the BVH-NR activity on the enzyme complex. Inactivation may involve reduction of NR. Reversibly inactivated NR can be reactivated in vitro by pre-treatment with FeCN. Inactivation is promoted by darkness or ammonium treatment.

In Chlamydomonas reinhardtii, reversibly inactivated NR is the main target for a protein degradation system; nitrate reductase related diaphorase must be integrated in a reversibly inactive nitrate reductase complex to undergo degradation (Franco et al, 1987). Similar mechanisms may operate in higher plants where proteinaceous inactivating factors and reversible inactivation of NR have been documented.

Nitrate is not absolutely required for induction of NR in green algae (e.g. NR is "derepressed" when cells are transferred from ammonium to a N-free medium). NR decay occurs upon transfer to ammonium or N-free medium.

Ammonium-induced decay of NR seems to depend on protein synthesis and is prevented by nitrate. Ammonium generally leads to an abrupt inhibition of nitrate uptake, possibly mediated by a product of ammonium assimilation (e.g. glutamine).

Fernandez E, Cardenas J 1989 Genetics and regulatory aspects of nitrate assimilation in algae. In "Molecular and Genetic Aspects of Nitrate Assimilation" (JL Wray, JR Kinghorn eds), Oxford Science Publications, Oxford, pp. 101-124.

Franco AR, Cardenas J, Fernandez E 1987 Involvement of reversible inactivation in the regulation of nitrate reductase enzyme levels in Chlamydomonas reinhardtii. Plant Physiol. 84: 665-669.