Mammalian p53R2 protein forms an active ribonucleotide reductase in vitro with the R1 protein, which is expressed both in resting cells in response to DNA damage and in proliferating cells

Recently, a homologue of the small subunit of mammalian ribonucleotide redu ctase (RNR) was discovered, called p53R2. Unlike the well characterized S p hase-specific RNR R2 protein, the new form was induced in response to DNA d amage by the p53 protein. Because the R2 protein is specifically degraded i n late mitosis and absent in G(0)/G(1) cells, the induction of the p53R2 pr otein may explain how resting cells can obtain deoxyribonucleotides for DNA repair. However, no direct demonstration of RNR activity of the p53R2 prot ein was presented and furthermore, no corresponding RNR large subunit was i dentified. In this study we show that recombinant, highly purified human an d mouse p53R2 proteins contain an iron-tyrosyl free radical center, and bot h proteins form an active RNR complex with the human and mouse R1 proteins. UV irradiation of serum-starved, G(0)/G(1)-enriched mouse fibroblasts, sta bly transformed with an RI promoter-luciferase reporter gene construct, cau sed a 3-fold increase in luciferase activity 24 h after irradiation, parall eled by an increase in the levels of RI protein. Taken together, our data i ndicate that the RI protein can function as the normal partner of the p53R2 protein and that an R1-p53R2 complex can supply resting cells with deoxyri bonucleotides for DNA repair.