Regulation of nitrate uptake in Nostoc muscorum by glutamine synthetase

Abstract

The assimilation of nitrate involves a nitrate reductase and a nitrite reductase which together catalyse the stepwise reduction of nitrate through nitrite to ammonium, in the ultimate conversion of inorganic nitrogen to L-glutamic acid. This process is supposed to be the predominant biological one for the production of reduced nitrogen from oxidized inorganic precursors, yet its regulation is not fully understood. Nitrate assimilation is repressed when adequate levels of the end product, ammonium, are present in the immediate environment. When both the nitrogen sources, ammonium and nitrate are available for the growth of the alga N ostoc muscorum, a preferential utilization of ammonium ions over nitrate takes place [I]. These results were explained by an inactivation of nitrate reductase [2]. In contrast, reports are available on the simultaneous uptake of nitrate and ammonium [3,4]. The observation that nitrate reductase activity in vitro is not affected by the presence of ammonium led to the conclusion that the enzyme is repressed by assimilation products derived from ammonium [5,6]. Now a consensus is evolving that recognizes the importance of the uptake step in the reduction of nitrate [7-10]. The possibility that ammonium inhibition of nitrate assimilation pathway is exercised first at the level of uptake rather than reduction has been suggested by several studies [11-14]. In blue-green algae such as Nostoc muscorum, there are no vacuoles to store nitrate ions in the usual way. Thus reduction of nitrate immediately follows its uptake making a distinction of these two processes difficult. The separation of nitrate uptake from its subsequent reduction may be helpful in obtaining information about the regulation by ammonium of the process. In this paper, we report that in Nostoc muscorum during short-term experiments, nitrate uptake is free from its subsequent reduction and the regulation of nitrate uptake occurs primarily at the level of glutamine synthetase activity. Product of the glutamine synthetase-catalysed reaction (the feedback system) merits further attention.