The purified 2-oxoacid:ferredoxin oxidoreductase of a thermoacidophilic and aerobic crenarchaeote, Sulfolobus sp. strain 7, consists of 70-kDa alpha and 37-kDa beta subunits, and contains one thiamine pyrophosphate (TPP), one [4Fe-4S]2+.1+ cluster, and two magnesium atoms per alpha beta structure. It exhibits a broad substrate specificity toward 2-oxoacids such as 2-oxoglutarate, 2-oxobutyrate, and pyruvate. The gene encoding the archaeal oxidoreductase was cloned, and the two open reading frames encoding the alpha (632 amino acids) and beta subunits (305 amino acids), respectively, were sequenced. Careful sequence alignment revealed several consensus motifs of this enzyme family, as well as possible cofactor binding residues of the Sulfolobus enzyme. This new structural information also indicates that (i) several genetic fusions and reorganization of the early, possibly alpha beta gamma delta-type enzyme similar to those from hyperthermophiles have taken place during evolution of the 2-oxoacid:ferredoxin (flavodoxin) oxidoreductase superfamily, which might have occurred in different ways in early aerobic archaea and early anaerobic bacteria, and that (ii) enzymes with different subunit compositions should have an essentially similar catalytic mechanism with one TPP and at least one [4Fe-4S] cluster as the minimal set of redox centers.