Transport of L-Glutamate across the Bacteroid Membrane but Not the Peribacteroid Membrane from Soybean Root Nodules

Uptake of L-[U- 1 4 C] glutamate by both peribacteroid membrane-enclosed bacteroids (peribacteroid units) and free bacteroids from soybean root nodules was studied. Free Bradyrhizobium japonicum USDAllO bacteroids were able to accumulate L-glutamate rapidly via a high affinity transport system (Km = 0.8 µM, Vmax = 23 nmoles.min^-1.mg protein^-1). However, the peribacteroid membrane prohibited supply of L­glutamate to enclosed bacteroids. The bacteroid transport system appeared to be relatively specific for carboxy-amino acids, as judged by inhibitor sensitivity, and exhibited a broad pH optimum around pH 6.5. The transport system was not stimulated by ATP or Na ions. Bacteroid L-glutamate uptake was, however, inhibited by uncoupler and respiratory poisons. We conclude that, although bacteroids have the potential to accumulate L-glutamate rapidly, the peribacteroid membrane is essentially impermeable to L­glutamate. The results suggest that L-glutamate does not play a role in carbon supply to the bacteroid during nitrogen fixation.