Nitrogen-fixing bacteroids in Legume root nodules are surrounded by the plant-derived peribacteroid membrane, which controls nutrient transfer between the symbionts, A nodule complementary DNA (GmSAT1) encoding an ammonium transporter has been isolated from soybean. GmSAT1 is preferentially transcribed in nodules and immunoblotting indicates that GmSAT1 is Located on the peribacteroid membrane. [C-14]methylammonium uptake and patch-clamp analysis of yeast expressing GmSAT1 demonstrated that it shares properties with a soybean peribacteroid membrane NH4+ channel described elsewhere. GmSAT1 is likely to be involved in the transfer of fixed nitrogen from the bacteroid to the host.