Part of a membrane-bound complex that couples electron transfer with translocation of ions across the membrane (By similarity). Required for nitrogen fixation. Stabilizes RnfB (PubMed:9154934). Binds 2 [4Fe-4S] clusters per subunit. The complex is composed of six subunits: RnfA, RnfB, RnfC, RnfD, RnfE and RnfG. Belongs to the 4Fe4S bacterial-type ferredoxin family. RnfC subfamily.