RNR4P, A NOVEL RIBONUCLEOTIDE REDUCTASE SMALL-SUBUNIT PROTEIN

Ribonucleotide reductases catalyze the formation of deoxyribonucleotid es by the reduction of the corresponding ribonucleotides. Eukaryotic r ibonucleotide reductases are alpha(2) beta(2) tetramers; each of the l arger, alpha subunits possesses binding sites for substrate and allost eric effecters, and each of the smaller, beta subunits contains a binu clear iron complex, The iron complex interacts with a specific tyrosin e residue to form a tyrosyl free radical which is essential for activi ty. Previous work has identified two genes in the yeast Saccharomyces cerevisiae, RNR1 and RNR3, that encode alpha subunits and one gene, RN R2, that encodes a beta subunit. Here we report the identification of a second gene from this yeast, RNR4, that encodes a protein with signi ficant similarity to the beta-subunit proteins. The phenotype of rnr4 mutants is consistent with that expected for a defect in ribonucleotid e reductase; rnr4 mutants are supersensitive to the ribonucleotide red uctase inhibitor hydroxyurea and display an S-phase arrest at their re strictive temperature, rnr4 mutant extracts are deficient in ribonucle otide reductase activity, and this deficiency can be remedied by the a ddition of exogenous Rnr4p. As is the case for the other RNR genes, RN R4 is induced hg agents that damage DNA. However, Rnr4p lacks a number of sequence elements thought to be essential for iron binding, and mu tation of the critical tyrosine residue does not affect Rnr4p function , These results suggest that Rnr4p is catalytically inactive but, none theless, does play a role in the ribonucleotide reductase complex.