Yeast Sml1, a protein inhibitor of ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides t o deoxyribonucleotides; this step is rate-limiting in DNA precursor synthes is. A number of regulatory mechanisms ensure optimal deoxyribonucleotide po ols, which are essential for cell viability. The best studied mechanisms ar e 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 regul ation has been hypothesized in yeast. A novel protein, Sml1, was shown to b ind to the Rnr1 protein of the yeast ribonucleotide reductase; this interac tion was proposed to inhibit ribonucleotide reductase activity when DNA syn thesis 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 mu M. Interestingly, Sml1p also specifically b inds to the mouse ribonucleotide reductase R1 protein. However, the inhibit ion observed in an in vitro mouse ribonucleotide reductase assay is less pr onounced than the inhibition in yeast and probably occurs via a different m echanism.