Inhibition of Assimilatory Nitrate Uptake by Ammonium Ions in Azorhizobium caulinodans IRBG 46

Addition of ammonium sulphate at low concentrations to Azorhizobium caulinodans IRBG 46 cells caused an immediate cessation of nitrate uptake activity, which was restored when the added ammonium ions were exhausted from the medium. Blockage of ammonium assimilation by L-methionine sulfoximine did not prevent the negative effect of ammonium on the assimilatory nitrate uptake, thus indicating that ammonium ions per se and not its assimilatory product(s) are actual regulators of assimilatory nitrate uptake.