THE TUNGSTEN FORMYLMETHANOFURAN DEHYDROGENASE FROM METHANOBACTERIUM-THERMOAUTOTROPHICUM CONTAINS SEQUENCE MOTIFS CHARACTERISTIC FOR ENZYMESCONTAINING MOLYBDOPTERIN DINUCLEOTIDE

Furmylmethanofuran dehydrogenases are molybdenum or tungsten iron-sulf ur proteins containing a pterin dinucleotide cofactor. We report here on the primary structures of the four subunits FwdABCD of the tungsten enzyme from Methanobacterium thermoautotrophicum which were determine d by cloning and sequencing the encoding genes fwdABCD. FwdB was found to contain sequence motifs characteristic for molybdopterin-dinucleot ide-containing enzymes indicating that this subunit harbors the active site. FwdA, FwdC and FwdD showed no significant sequence similarity t o proteins in the data bases. Northern blot analysis revealed that the four fwd genes form a transcription unit together with three addition al genes designated fwdE, fwdF and fwdG. A 17,8-kDa protein and an 8.6 -kDa protein, both containing two [4Fe-4S] cluster binding motifs, wer e deduced from fwdE and fwdG., The open reading frame fwdF encodes a 3 8.6-kDa protein containing eight binding motifs for [4Fe-4S] clusters suggesting the gene product to be a novel polyferredoxin. All seven fo ld genes were expressed in Escherichia coli yielding proteins of the e xpected size. The fwd operon was found to be located in a region of th e M. thermoautotrophicum genome encoding molybdenum enzymes and protei ns involved in molybdopterin biosynthesis.