Ribonucleotide reductase R1 gene expression is elevated in BALB/c 3T3 fibroblasts treated with the tumor promoter, 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA). We show that TPA treatment increased the half‐life of R1 mRNA by 5‐fold, indicating that TPA regulates R1 gene expression by a post‐transcriptional mechanism. We investigated the possibility that the 3′ untranslated region (3′UTR) of R1 mRNA contains regulatory information for TPA‐mediated message stability. Our studies demonstrated that a 49 nucleotide (nt) TPA‐responsive region existed within the R1 mRNA 3′UTR. Deletion of the 49 nt region led to the abolishment of TPA‐induced stability of R1 and hybrid CAT mRNAs. Further deletions of the 3′UTR did not significantly affect mRNA turnover rates. In addition, we detected by cross‐linking a novel 52–57 kDa R1 mRNA‐binding protein (R1BP) that bound selectively to the 49 nt region of the R1 mRNA 3′UTR and did not bind to the 5′UTR, the coding region or other mRNA 3′UTRs. The R1BP‐RNA binding activity observed in unstimulated cells was rapidly and markedly down‐regulated after TPA treatment, suggesting a role for R1BP in mRNA metabolism, and in the mechanism of action of TPA‐induced R1 message stabilization. These results support a novel model of R1 gene regulation in which a cis‐element(s) within the 49 nt region of the R1 mRNA 3′UTR interacts with R1BP in a mechanism that regulates R1 message stability. We propose that this model accounts for the TPA‐mediated stability alteration of R1 message, through down‐regulation of R1BP‐RNA binding activity linked to a reduction in the rate of R1 mRNA degradation.

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