Effect of Nitrogen Supply on the Kinetics and Regulation of Nitrate Assimilation in Chlamydomonas reinhardtii Dangeard

Abstract

Concentration-dependent NO3 uptake by C. reinhardtii was studied in cells cultured under three NO3-growth regimes: 10 mM NO3 (NOJ -grown), 0-25 mM NO3 (N-limited) and an 18 h period of N-deprivation following growth on 10 mM NO J (N-starved). Two NO3 uptake isotherms, with an apparent phase transition in uptake at IT mM NO3, were resolved in all three cell types over the concentration range tested (0025 to 1-8 mM N03). The apparent Kmax for uptake by N-limited cells (0-72 x 10-8 /.?mol cell-1 h-1) exceeded that for both NO3-grown (0-42 x 10-8 /??mol cell-1 h-1) and N-starved (0-49 x 10-8 /??mol cell-1 h-1) cells at NO3 concentrations less than IT mM (isotherm 0). When supplied with NOj concentrations in excess of IT mM (isotherm 1) the apparent Kmax values for uptake by N-limited (0-64 x 10-8 /?mol cell-1 h-1) and N-starved (0-41 x 10-8 /?mol cell-1 h-1) cells were depressed relative to those for isotherm 0. In contrast, NO3-grown cells exhibited a greater apparent Vmax for uptake (0-63 x 10-8/?mol cell-1 h-1) at the higher NO3 concentrations. When N-limited and N-starved cells were supplied with 1-6 mM NO J (isotherm 1), the pattern of uptake showed a rapid linear increase after 60 min. This enhanced pattern of uptake did not occur in NO J-grown cells supplied with either 0-6 or 1-6 mM NO3 or in N-limited and N-starved cells supplied with 0-6 mM NO J. Cycloheximide (10 /?g cm-3), but not actinomycin D (50 /?g cm-3), blocked the apparent 'induction' of NO3 uptake in N-starved cells supplied with l-6mM NO J. These results suggest that mechanisms controlling the observed 'inducible' aspect of NO3 uptake in N-starved cells are dependent on a translational or post translational event. It is possible that the NO3 -concentration dependent 'inducible' portion of uptake may provide the means by which C. reinhardtii cells may regulate uptake in accordance with their capacity for assimilation.