ALLOSTERIC CONTROL OF THE SUBSTRATE-SPECIFICITY OF THE ANAEROBIC RIBONUCLEOTIDE REDUCTASE FROM ESCHERICHIA-COLI

The reduction of ribonucleotides is catalyzed by different enzymes in aerobic and anaerobic Escherichia coli, each with a different primary and quaternary structure. Here, we describe the allosteric regulation of the substrate specificity of the anaerobic ribonucleoside triphosph ate reductase. The enzyme reduced ribonucleotides at a low basal rate. Reduction was stimulated up to 10-fold by an appropriate modulator (d GTP for ATP reduction, ATP for CTP and UTP reduction, and dTTP for GTP reduction). dGTP and dTTP inhibited the reduc tion of the ''incorrect '' substrate; dATP inhibited reduction of all four. From kinetic, effe ctor binding, and competition experiments we conclude that the enzyme has two classes of sites, one that binds ATP and dATP and regulates py rimidine ribonucleotide reduction (''pyrimidine site''), the other tha t binds dATP, dGTP, and dTTP and regulates purine ribonucleotide reduc tion (''purine site''). This model differs slightly from the model for the aerobic reductase, but the physiological consequences remain the same and explain how a single enzyme can provide a balanced supply of the four dNTPs. The similarity of a highly sophisticated control mecha nism for the aerobic and anaerobic enzymes suggests that both arose by divergent evolution from a common ancestor, in spite of their differe nt structures.