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-’ (K_m=11μM, V_max=0.22nmol/min/mg protein) and ‘low-affinity’ (K_m=66μM, V_max=1.25nmol/min/mg protein). Both transport systems were competitively inhibited by methylamine (high-affinity K_i=20μM; low affinity K_i=80μM), and showed distinct pH profiles. Addition of Cu²⁺ (0.1μM) stimulated NH₄⁺ uptake by the high-affinity system (K_m=8μM, V_max=0.42nmol/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-chloromercuribenzoate) 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.