Formylmethanofuran dehydrogenases arc molybdenum or tungsten iron-sulfur proteins containing a pterin dinucleolide cofactor. We report here on the primary structures of the four subunits FwdABCD of the tungsten enzyme from Methmohucteriuni thermoautcltrrii~hic.unz which were dctermincd by cloning and sequencing the encoding genes jwdABCD. FwdB was found to contain sequence motifs characteristic for molybdopterin-dinucleotidc-containing cnzytncs indicating that this subunit harbors the active site. FwdA, FwdC and FwdD showed no significant sequcnce similarity lo proteins in the diila bases. Northern blot analysis revealed that the four j w d genes form a transcription unit together with three additional genes designatedfwdE,fwdF andfwdC. A 17.8-kDa protein and an 8.6-kDa protein, both containing two 14Fe-4SI cluster binding motifs, were deduced frotii f w d E and fwdC. The open reading frame ,fivdF encodes a 38.6-kDa protein containing eight binding motifs for [4Fe-4S] clusters suggesting the gene product to be a novel polyferredoxin. All seven fwtl gcnes were expresscd i n Esclwrichiu coli yielding proteins of the expecled size. Thefwd operon was found to be located in a region of the M. thernroautotruphicum genome encoding molybdenum enzymes and proteins involved i ti iiiolybdopterin hiosynthesis.Kt?ywurds: methanogenic Archaea; tungsten enzymes; molybdenum enzymes; molybdopterin; iron-sulfui proteins.Formylmethanofuran dchydrogenasc is an enzyme found in methanogenic and sulfate-reducing Archaea. It catalyzes the reversible dehydrogenation of formylmethanofuran to CO, and methanofuran (E" = -530 mV). The reaction is iiivolved in C 0 7 reduction to methane, in autotrophic CO, fixation, and i n CO, formation from reduced C, units (Bertram and Thauer, 1994;Wasserfallen, 1994; Vorholt ct al., 1995).Fortnylrnetlianofuran dehydrogenases are either molybdenum or tungsten iron-sulfur proteins. The tungsten seems to be bound to the satiic skeleton as the molybdenum in the so-callcd molybdopterin djnucleotide cofactor (Karrasch et al., 1990; Bijrner et al., 1991). In Methanobacterium wo(fei and Mr,thanobmcterium thermoaututrnphicum two isoenzymes of formylmethanofuran dehydrogenase are found, one containing molybdenum and the other tungsten (Schmitz ct al., 1992a,b; Bertram et al., 1994a,b). In M. thermoautotrophicum the active molybdenum isoenzyme is only synthesized when molybdenum is available during growth. The active tungsten enzyme is also generated during growth of the cells on molybdate medium. Under
___-Fmd, molybdenum I'ormylmcthanofuran dehydrogenase. the latter conditions the tungstcn isocnzyme is synthesized containing mvlybdenutn rather than tungsten.The tungstcn isoenzynie of M. thermooutotro/ihicum is composed of four differenl subunits of apparent niolecular masses of65 kDa (FwdA), 53 kDa (FwdB), 31 kDa (FwdC) and IS kDa (FwdD). It contains 0.4 inol tungsten, 0.6 in01 molybdopterin guanine dinucleotide and approximatcly X inol nun-heme iron and acid-labilc sulfur/mol 160-kDa native ewytiic. Thc molyb...