Ribonucleotide reductases (RNRs) convert ribonucleotides to deoxynucleotides, a process essential for DNA biosynthesis and repair. Class Ia RNRs require two dimeric subunits for activity: an α2 subunit that houses the active site and allosteric regulatory sites and a β2 subunit that houses the diferric tyrosyl radical cofactor. Ribonucleotide reduction requires that both subunits form a compact α2β2 state allowing for radical transfer from β2 to α2 RNR activity is regulated allosterically by dATP, which inhibits RNR, and by ATP, which restores activity. For the well-studied Escherichia coli class Ia RNR, dATP binding to an allosteric site on α promotes formation of an α4β4 ring-like state. Here, we investigate whether the α4β4 formation causes or results from RNR inhibition. We demonstrate that substitutions at the α-β interface (S37D/S39A-α2, S39R-α2, S39F-α2, E42K-α2, or L43Q-α2) that disrupt the α4β4 oligomer abrogate dATP-mediated inhibition, consistent with the idea that α4β4 formation is required for dATP's allosteric inhibition of RNR. Our results further reveal that the α-β interface in the inhibited state is highly sensitive to manipulation, with a single substitution interfering with complex formation. We also discover that residues at the α-β interface whose substitution has previously been shown to cause a mutator phenotype in Escherichia coli (i.e. S39F-α2 or E42K-α2) are impaired only in their activity regulation, thus linking this phenotype with the inability to allosterically down-regulate RNR. Whereas the cytotoxicity of RNR inhibition is well-established, these data emphasize the importance of down-regulation of RNR activity.