Part of a membrane-bound complex that couples electron transfer with translocation of ions across the membrane. Couples electron transfer from reduced ferredoxin to NAD(+) with translocation of H(+) out of the cell. Essential for energy conservation during autotrophic growth. Contributes to ATP synthesis during heterotrophic growth. Binds 5 [4Fe-4S] clusters. The complex is composed of six subunits: RnfA, RnfB, RnfC, RnfD, RnfE and RnfG. Belongs to the 4Fe4S bacterial-type ferredoxin family. RnfB subfamily.