Part of a membrane-bound complex that couples electron transfer with translocation of ions across the membrane (By similarity). Required for nitrogen fixation. Involved in electron transfer to nitrogenase (PubMed:8264535). Binds 3 [4Fe-4S] clusters. The complex is composed of six subunits: RnfA, RnfB, RnfC, RnfD, RnfE and RnfG. Expression is reduced under iron-limiting conditions. Belongs to the 4Fe4S bacterial-type ferredoxin family. RnfB subfamily.