AMMONIUM TRANSPORT IN NITROGEN-FIXING CYANOBACTERIUM NOSTOC MUSCORUM: INTERACTION WITH COPPER AND SULFHYDRYL AGENTS

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

[学科分类] 微生物学和免疫学