Modulation of nitrate uptake in Anacystis nidulans by the balance between ammonium assimilation and CO2 fixation

Gradual inhibition of ammonium assimilation in Anacystis nidulans cells by increasing concentrations of 5-hydroxylysine resulted in a progressive enhancement of nitrate uptake. For 5-hydroxylysine-treated cells, the magnitude of the inhibition of nitrate uptake promoted by added ammonium was dependent on the ammonium assimilation capacity. In cells with a moderate ammonium assimilation activity, acceleration of CO2 fixation induced by bicarbonate addition antagonized the negative effect of ammonium, allowing full nitrate uptake activity. The results support the contention that nitrate utilization is under the feed-back control exerted by products of its own assimilation via ammonium, the inhibitory effect being potentiated by ammonium addition and alleviated by enhanced CO2 fixation. Results of amino acid analysis in cells exhibiting different capacities to utilize nitrate speak against these compounds as direct effectors of nitrate uptake.