Characterization of the cdhD and cdhE genes encoding subunits of the corrinoid/iron-sulfur enzyme of the CO dehydrogenase complex from Methanosarcina thermophila

The CO dehydrogenase enzyme complex from Methanosarcina thermophila contains a corrinoid/iron-sulfur enzyme composed of two subunits (δ and γ). The cdhD and cdhE genes, which encode the δ and γ subunits, respectively, were cloned and sequenced. The cdhD gene is upstream of and separated by 3 bp from cdhE. Both genes are preceded by apparent ribosome-binding sites. Northern (RNA) blot and primer extension analyses indicated that cdhD and cdhE are cotranscribed from a promoter located several kilobases upstream of cdhD. The putative CdhD and CdhE sequences are 37% identical to the sequences deduced from the genes encoding the β and α subunits of the corrinoid/iron- sulfur enzyme from Clostridium thermoaceticum. The CdhE sequence had a four- cysteine motif with the potential to bind a 4Fe-4S cluster previously identified in the corrinoid/iron-sulfur enzyme by electron paramagnetic resonance spectroscopy. A T7 RNA polymerase/promoter system was used to produce CdhD and CdhE independently in Escherichia coli. The purified CdhD protein was reconstituted with hydroxocobalamin in the base-on configuration. The purified CdhE protein exhibited an Fe-S center and base-off cobalamin binding in which the benzimidazole base nitrogen atom was no longer a lower axial ligand to the cobalt atom.