Structural interconversions modulate activity of Escherichia coli ribonucleotide reductase

Essential for DNA biosynthesis and repair, ribonucleotide reductases (RNRs) convert ribonucleotides to deoxyribonucleotides via radical-based chemistry. Although long known that allosteric regulation of RNR activity is vital for cell health, the molecular basis of this regulation has been enigmatic, largely due to a lack of structural information about how the catalytic subunit (α ₂) and the radical-generation subunit (β ₂) interact. Here we present the first structure of a complex between α ₂ and β ₂ subunits for the proto-typic RNR from Escherichia coli. Using four techniques (small-angle X-ray scattering, X-ray crystallography, electron microscopy, and analytical ultracentrifugation), we describe an unprecedented α ₄β ₄ring-like structure in the presence of the negative activity effector dATP and provide structural support for an active α ₂β ₂ configuration. We demonstrate that, under physiological conditions, E. coli RNR exists as a mixture of transient α ₂β ₂ and α ₄β ₄ species whose distributions are modulated by allosteric effectors. We further show that this interconversion between α ₂β ₂ and α ₄β ₄ entails dramatic subunit rearrangements, providing a stunning molecular explanation for the allosteric regulation of RNR activity in E. coli.