After activation with NiCl2, the recombinant α subunit of the Ni-containing α2β2 acetyl-CoA synthase/carbon monoxide dehydrogenase (ACS/CODH) catalyzes the synthesis of acetyl-CoA from CO, CoA, and a methyl group donated from the corrinoid-iron-sulfur protein (CoFeSP). The α subunit has two conformations (open and closed), and contains a novel [Fe4S 4]-[Nip Nid] active site in which the proximal Nip ion is labile. Prior to Ni activation, recombinant apo-α contain only an Fe4S4 cluster. Ni-activated α subunits exhibit catalytic, spectroscopic and heterogeneity properties typical of α subunits contained in ACS/CODH. Evidence presented here indicates that apo-α is a monomer whereas Ni-treated α oligomerizes, forming dimers and higher molecular weight species including tetramers. No oligomerization occurred when apo-α was treated with Cu(II), Zn(II), or Co(II) ions, but oligomerization occurred when apo-α was treated with Pt(II) and Pd(II) ions. The dimer accepted only 0.5 methyl group/α and exhibited, upon treatment with CO and under reducing conditions, the NiFeC EPR signal quantifying to 0.4 spin/α. Dimers appear to consist of two types of α subunits, including one responsible for catalytic activity and one that provides a structural scaffold. Higher molecular weight species may be similarly constituted. It is concluded that Ni binding to the A-cluster induces a conformational change in the α subunit, possibly to the open conformation, that promotes oligomerization. These interrelated events demonstrate previously unrealized connections between (a) the conformation of the α subunit; (b) the metal which occupies the proximal/distal sites of the A-cluster; and (c) catalytic activity.