IDENTIFICATION OF A TRANSPORT MECHANISM FOR NH4-NODULES( IN THE SYMBIOSOME MEMBRANE OF PEA ROOT)

Symbiosome membrane vesicles, facing bacteroid-side-out, were purified from pea (Pisum sativum L.) root nodules and used to study NH4+ trans port across the membrane by recording vesicle uptake of the NH4+ analo g [C-14]methylamine (MA). Membrane potentials (Delta psi) were imposed on the vesicles using K+ concentration gradients and valinomycin, and the size of the imposed Delta psi was determined by measuring vesicle uptake of [C-14]tetraphenylphosphonium. Vesicle uptake of MA was driv en by a negative Delta psi and was stimulated by a low extravesicular pH. Protonophore-induced collapse of the pH gradient indicated that up take of MA was not related to the presence of a pH gradient. The MA-up take mechanism appeared to have a large capacity for transport, and sa turation was not observed at MA concentrations in the range of 25 mu M to 150 mM. MA uptake could be inhibited by NH4+, which indicates that NH4+ and MA compete for the same uptake mechanism. The observed fluxe s suggest that voltage-driven channels are operating in the wsymbiosom e membrane and that these are capable of transporting NH4+ at high rat es from the bacteroid side of the membrane to the plant cytosol. The p H of the symbiosome space is likely to be involved in regulation of th e flux.