Yeast Sml1, a protein inhibitor of ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides; this step is rate-limiting in DNA precursor synthesis. A number of regulatory mechanisms ensure optimal deoxyribonucleotide pools, which are essential for cell viability. The best studied mechanisms are transcriptional regulation of the RNR genes during the cell cycle and in the response to DNA damage, and the allosteric regulation of ribonucleotide reductase by nucleoside triphosphates. Recently, another mode of RNR regulation has been hypothesized in yeast. A novel protein, Sml1, was shown to bind to the Rnr1 protein of the yeast ribonucleotide reductase; this interaction was proposed to inhibit ribonucleotide reductase activity when DNA synthesis is not required (Zhao, X., Muller, E.G.D., and Rothstein, R. (1998) Mol. Cell 2, 329-340). Here, we use highly purified recombinant proteins to directly demonstrate that the Sml1 protein is a strong inhibitor of yeast RNR. The Sml1p specifically binds to the yeast Rnr1p in a 1:1 ratio with a dissociation constant of 0.4 microM. Interestingly, Sml1p also specifically binds to the mouse ribonucleotide reductase R1 protein. However, the inhibition observed in an in vitro mouse ribonucleotide reductase assay is less pronounced than the inhibition in yeast and probably occurs via a different mechanism.