Characteristics of Ammonium Transport in an Alkalophilic Diazotrophic Cyanobacterium Nostoc calcicola: Influence of Temperature and Methionine Sulfoximine

Abstract

Ammonium transport in nitrogen fixing cyanobacterium Nostoc calcicola showed the existence of two ammonium-specific transport systems: (a) high-affinity (Km = 10 µM, Vmax = 50 nmol · mg-1 protein · min-1), and (b) low-affinity (Km = 200 µM, Vmax = 500 nmol · mg-1 protein · min-1). Experiments on pH profile showed that the former system was maximally operative at pH 6.0 and the latter at pH 7.0. Following 10 min of NH4 uptake, 16- and 75-fold concentration gradients were formed by high- and low-affinity systems, respectively, and formation of an intracellular NH4+ pool was independent of glutamine synthetase activity. Investigations on temperature profile revealed that the optimal temperature for the high-affinity system was 30 °C and for the low-affinity system was 40 °C. Discontinuities in the slopes of the Arrhenius plot for the two transport systems suggested that a 2.5-fold higher activation energy was required for the low-affinity system than the high-affinity system. Shift-down (50° → 30 °C) experiments revealed that irreversible inactivation of the transport process occurred following exposure to critical temperature. Inhibition of NH4+ transport by higher temperature and chloramphenicol suggested involvement of a proteinaceous carrier(s) in the process. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.