The roles of nitrate and ammonium in the regulation of the development of nitrate reductase in Chlamydomonas reinhardii

Abstract

The regulation of the development of nitrate reductase (NR) activity in Chlamydomonas reinhardii has been compared in a wild-type strain and in a mutant (nit-A) which possesses a modified nitrate reductase enzyme that is non-functional in vivo. The modified enzyme cannot use NAD(P)H as an electron donor for nitrate reduction and it differs from wild-type enzyme in that NR activity is not inactivated in vitro by incubation with NAD(P)H and small quantities of cyanide; it is inactivated when reduced benzyl viologen or flavin mononucleotide is present. After short periods of nitrogen starvation mutant organisms contain much higher levels of terminal-NR activity than do similarly treated wild-type ones. Despite the inability of the mutant to utilize nitrate, no nitrate or nitrite was found in nitrogen-starved cultures; it is therefore concluded that the appearance of NR activity is not a consequence of nitrification. After prolonged nitrogen starvation (22 h) the NR level in the mutant is low. It increases rapidly if nitrate is then added and this increase in activity does not occur in the presence of ammonium, tungstate or cycloheximide. Disappearance of preformed NR activity is stimulated by addition of tungstate and even more by addition of ammonium. The results are interpreted as evidence for a continuous turnover of NR in cells of the mutant with ammonium both stimulating NR breakdown and stopping NR synthesis. Nitrate protects the enzyme from breakdown. Reversible inactivation of NR activity is thought to play an insignificant rôle in the mutant.

Characterization of Chlamydomonas reinhardtii phosphatidylglycerophosphate synthase in Synechocystis sp. PCC 6803

“…These results showed that expression profiles of CrPGP1 and CrPGP2 , as well as other related genes examined in Figure 7 , are altered under phosphorus starvation. Next, we examined nitrogen starvation using CrNIT1 as a marker gene ( Hipkin et al, 1980 ). This condition severely affects photosynthesis but induces accumulation of triacylglycerol ( Wang et al, 2009 ).…”

Regulation of the absorption and release of nitrate by plant cells: A review of current ideas and methodology

“…It is clear, however, that during starvation there is a second process initiated which tends to increase the level of the carrier, perhaps this might be regarded as bring up the constitutive level of the carrier. These separate processes have been described in various algae -in C. corallina '8 in Chlamydomonas reinhardii 38 and in Chlarella 31 . To these two processes can be added a third, viz the rapid decline in growth rate when nitrogen supply becomes sub-optimal which reduces the demand for nitrate 39 . These opposing tendencies can sometimes give rise to puzzling changes in the net uptake of nitrate when plants have been temporarily deprived of external nitrate; some data from Arabidopsis 25 can be used to illustrate this point (Fig.…”

Regulation of nitrate assimilation in the cyanobacterium Phormidium laminosum

“…Unlike other cyanobacteria (Herrero et al 1981;Herrero and Guerrero 1986), algae (Hipkin et al 1980) and higher plants (Redinbaugh and Campbell 1991), P. laminosum responded to the shortage of nitrogen assimilation products by substantially increasing the activity of the enzymes involved in its assimilation. To our knowledge, this is the only cyanobacterium known to date that shows higher NR and NiR activities in N-starved than in nitrate-growing cells.…”