Part of a membrane-bound complex that couples electron transfer with translocation of ions across the membrane (By similarity). Required for nitrogen fixation. Necessary for stable existence of both RnfB and RnfC (PubMed:9154934). The complex is composed of six subunits: RnfA, RnfB, RnfC, RnfD, RnfE and RnfG. Belongs to the NqrDE/RnfAE family.