Ribonucleotide reductases (RNRs) reduce ribonucleotides to deoxyribonucleotides. Their overall activity is stimulated by ATP and downregulated by dATP via a genetically mobile ATP cone domain mediating the formation of oligomeric complexes with varying quaternary structures. The crystal structure and solution X-ray scattering data of a novel dATP-induced homotetramer of the Pseudomonas aeruginosa class I RNR reveal the structural bases for its unique properties, namely one ATP cone that binds two dATP molecules and a second one that is non-functional, binding no nucleotides. Mutations in the observed tetramer interface ablate oligomerization and dATP-induced inhibition but not the ability to bind dATP. Sequence analysis shows that the novel type of ATP cone may be widespread in RNRs. The present study supports a scenario in which diverse mechanisms for allosteric activity regulation are gained and lost through acquisition and evolutionary erosion of different types of ATP cone.