Ammonium Transport in Unicellular Cyanobacterium Anacystis nidulans

Abstract

Summary Anacystis nidulans IU 625, grown on nitrate, has been shown to possess two ammonium (NH + 4 ) uptake systems; the high- (K m = 50 μM, V max = 2 nmol min −1 mg −1 protein) and low-affinity (K m = 357 μM, V max = 10 nmol min −1 mg −1 protein) systems. The two uptake systems were specific for NH + 4 since methylamine competitively inhibited both the uptake systems (high-affinity K 1 = 100 μM, low-affinity K i = 833 μM). The pH profile of the uptake systems suggested that the high-affinity system was maximally operative at pH 7.4 and the low affinity system at pH 5.2. The low affinity system was suppressed in cells grown on nitrate or ammonium while the high-affinity system was not completely repressed (52 and 36 % respectively). Both the transport systems were light-dependent and sensitive to DCMU. Effect of other metabolic inhibitors (DCCD, HOQNO, NEM) revealed that energy generated as a result of ATP hydrolysis was more important for NH + 4 uptake by the low-affinity system, whilst the high-affinity system was more sensitive to protonophore FCCP suggesting an involvement of proton gradient of membrane. Concentrations of MSX non-inhibitory to GS activity during the course of experimentation showed mixed type inhibition of NH + 4 uptake by the high-affinity system and non-competitive inhibition of the low-affinity system. Mutant A. nidulans /Mn 10 showed only one high-affinity system (K m = 250 μM, V max = 10 nmol min −1 mg −1 protein) maximally operative at pH 7.4, comparable to the lowaffinity system of the wild type. It is concluded that NH + 4 uptake in A. nidulans is genetically controlled.