FERREDOXIN-DEPENDENT REDOX SYSTEM OF A THERMOACIDOPHILIC ARCHAEON, SULFOLOBUS SP STRAIN-7 - PURIFICATION AND CHARACTERIZATION OF A NOVEL REDUCED FERREDOXIN-REOXIDIZING IRON-SULFUR FLAVOPROTEIN

To elucidate the ferredoxin-dependent redox system of the thermoacidop hilic, aerobic archaeon Sulfolobus sp. strain 7, a novel FeS flavoprot ein, 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 th e archaeal tricarboxylic acid cycle. IFP consists of three non-identic al subunits with apparent molecular masses of 87, 32, and 22 kDa, resp ectively, and contains at least two FMN (E(m),6.8 = -57 mV) and two pl ant-ferredoxin-type [2Fe-2S](2+,1+) clusters (E(m),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 ferredox in under anaerobic conditions at 50 degrees C, but not by NAD(P)H. Thu s, 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).