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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 /2M, Vmax=0.22 nmol/min/mg protein) and 'low-affinity' (Km==66 /AM, Vmax =1.25 nmol/min/mg protein). Both transport systems were competitively inhibited by methylamine (high-affinity Kz=20 , tM; low affinity Ki=80 µM), and showed distinct pH profiles. Addition of Cu2+ (0.1 , tM) 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+, Nit +, Zn2+, Mn2+) applied at equimolar concentrations. The sulfhydryl reducing agents, cysteine and dithiothreitol, inhibited the high-affinity system noncompetitively and caused effiux of accumulated NH4+. Cu2+ eliminated the inhibitory effect of sulf hydryl reducing agents on NH4+ uptake. Inhibition of NH4+ uptake by sulf hydryl blocking agents (N-ethylmaleimide or p-chloromercuribenzoate) which was not reversible by Cu2+ suggested that oxidation of available sulf hydryl residues of membrane proteins (carriers) is an important factor in NH4+ translocation in Nostoc muscorum.The free-living N2-fixing microorganisms are known to respond to the fixed form of nitrogen in the immediate environment by repressing nitrogenase synthesis. The nature of the regulatory mechanism is not well understood. However, a possibility exists, though unproved, that NH4+ uptake is one of the regulatory mechanisms which may be similar to catabolite repression resulting from transport of sugar across the inner membrane (1, 2). The existence of ammonia as NH4+ 267

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