Ammonium transport in nitrogen-fixing cyanobacterium nostoc muscorum: Interaction with copper and sulfhydryl agents

Abstract

Ammonium (NH4+) uptake in the cyanobacterium Nostoc muscorum ISU (Anabaena ATCC 27893) and interaction of copper (Cu2+) and sulfhydryl agents was studied. N2-grown cells scavenged extracellular NH4+via two energy-dependent transport systems: the ‘high-’ (Km= 11 μM, Vmax=0.22 nmol/min/mg protein) and ‘low-affinity’ (Km=66 μM, Vmax=1.25 nmol/min/mg protein). Both transport systems were competitively inhibited by methylamine (high-affinity Kt=20 μM; low affinity Kt=80 μM), and showed distinct pH profiles. Addition of Cu2+(0.1 μM) stimulated NH4+uptake by the high-affinity system (Km=8 μM, vmax= 0.42 nmol/min/mg protein). Similar effect was not observed with other bivalent cations (Hg2+, Ni2+, Zn2+, Mn2+) applied at equimolar concentrations. The sulfhydryl reducing agents, cysteine and dithiothreitol, inhibited the high-affinity system noncompetitively and caused efflux of accumulated NH4+. Cu2+eliminated the inhibitory effect of sulfhydryl reducing agents on NH4+uptake. Inhibition of NH4+uptake by sulfhydryl blocking agents (N-ethylmaleimide or p-chloromercuri-benzoate) which was not reversible by Cu2+suggested that oxidation of available sulfhydryl residues of membrane proteins (carriers) is an important factor in NH4+translocation in Nostoc muscorum. © 1984, Applied Microbiology, Molecular and Cellular Biosciences Research Foundation. All rights reserved.