Phorbol ester modulation of a novel cytoplasmic protein binding activity at the 3'-untranslated region of mammalian ribonucleotide reductase R2 mRNA and role in message stability.

The R2 gene of ribonucleotide reductase is elevated in BALB/c 3T3 fibroblasts treated with the tumor promotor, 12-O-tetradecanoylphorbol-13-acetate (TPA). TPA treatment increased the half-life of the R2 message by 3-fold, showing that TPA regulates R2 gene expression by a post-transcriptional mechanism(s). A 20-nucleotide (nt) TPA-responsive region was found within the R2 mRNA 3'-untranslated region (3'UTR). Ultraviolet cross-linking detected a novel 45-kDa protein-R2 mRNA complex migration band that bound selectively to the 20-nt fragment and did not bind to the 5'UTR or the coding region of the R2 message, or to the 3'UTRs of mRNA from several other genes, or to the homopolymer poly(A) sequence. The-45 kDa protein-R2 mRNA binding activity observed in unstimulated cells was markedly down-regulated after TPA treatment. Deletion of a 201-nt region, containing the 20-nt sequence, from the 3'UTR caused stabilization of hybrid chloramphenicol acetyltransferase mRNA in the absence of TPA treatment. Furthermore, in vitro decay reaction mixtures supplemented with the 20-nt sense RNA transcript resulted in stabilization of R2 message. A model is presented of R2 message regulation in which a cis-element within the 20-nt sequence of the 3'UTR interacts with a cytosolic protein to form a 45-kDa protein-mRNA binding complex. The TPA-induced alteration of R2 message stability is at least in part due to the down-regulation of the 45-kDa protein-mRNA binding activity which is linked to a reduction in the rate of R2 mRNA degradation.