Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 1/1/1998
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Nitrate reductase (NR) is the first and rate limiting step in nitrogen assimilation. The enzyme reduces nitrate to nitrate using NAD(P)H as the electron donor. NR activity (A) is under a circadian control thought to be a combination of protein level changes and covalent modification. This rhythm shifts under chilling conditions. By stopping and resuming upon rewarming the rhythm of enzyme activity effectively becomes uncoordinated with the ambient diurnal cycle. The mistiming of NR enzymatic activity could contribute to declines in photosynthetic rates observed in chilling sensitive species such as tomato. In vitro NR assays conducted with and without Mg**+2 exhibit a distinct circadian rhythm over a 70-hour time course. The presence of Mg**+2 in NR reactions allows the binding of an inhibitor protein needed to inactivate the enzyme. NRA rates without Mg**+2 are interpreted as protein turnover and the percent of NRA inhibited by +M**+2 should reflect activity changes due to inhibitor protein binding allowed by enzyme phosphorylation. In vitro assays of NRA with and without Mg**+2 have shown that a post-chill circadian rhythm shift does occur, and that rises in NRA in this rhythm are subject to inhibition by PP2A phosphatase inhibitors, translation inhibitors and transcription inhibitors. This implicates a role for circadian control of NRA by phosphatase transcription and translation. Northern bolt analysis indicates a corresponding rhythm in NR transcript levels.