To elucidate the ferredoxin-dependent redox system of the thermoacidophilic, aerobic archaeon Sulfolobus sp. strain 7, a novel FeS flavoprotein, which can reoxidize the reduced 7Fe ferredoxin in vitro, has been purified and characterized (designated as IFP) using the cognate 7Fe ferredoxin and 2-oxoacid:ferredoxin oxidoreductase, a key enzyme of the archaeal tricarboxylic acid cycle. IFP consists of three non-identical subunits with apparent molecular masses of 87, 32, and 22 kDa, respectively, and contains at least two FMN (Em, 6.8 = -57 mV) and two plant-ferredoxin-type [2Fe-2S]2+,1+ clusters (Em, 6.8 = -260 mV)/alpha 2 beta 2 gamma 2 structure. Both FeS and flavin centers of IFP are slowly but fully reduced by the enzymatically reduced cognate ferredoxin under anaerobic conditions at 50 degrees C, but not by NAD(P)H. Thus, the ferredoxin-dependent redox system of Sulfolobus sp. strain 7 is tentatively proposed as follows: 2-oxoacid:ferredoxin oxidoreductase (thiamine pyrophosphate and [4Fe-4S] cluster)-->ferredoxin-->IFP ([2Fe-2S] cluster-->FMN).