1. 1. In the presence of tungstate, nitrate reductase develops in a non-functional form. The development of the nitrate uptake system is not inhibited by tungstate. This makes it possible to measure nitrate uptake rates as nitrate accumulation rates. 2. 2. The nitrate uptake system of tobacco XD cells has a vmax of 2-5 μmoles nitrate/h per fresh wt. and an apparent Km for nitrate of 0.4 mM. It can concentrate nitrate 80-fold and the process is energy dependent. 3. 3. Development of an active nitrate uptake system is not initiated by nitrogen starvation nor by slow growth on a poor nitrogen source, urea. Nitrate specifically induces development of an active nitrate uptakes system. 4. 4. The rate of development of the nitrate uptake system is maximal immediately upon exposure of the cells to nitrate, before nitrate has accumulated appreciably. Upon removal of the exogenous supply of nitrate from fully induced cells, the accumulated nitrate is slowly assimilated and the nitrate reductase activity decays. The initially high concentrations of accumulated nitrate do not maintain nitrate reductase induction. It is assumed that distinct substrate and inducing pools of nitrate exist in the cells. 5. 5. Casein hydrolysate inhibits nitrate accumulation. Thus the nitrate uptake system in tobacco XD cells is subject to end product regulation by amino acids.
ASJC Scopus subject areas
- Molecular Biology