A rate-limiting reaction in DNA synthesis is catalyzed by ribonucleotide reductase, the enzyme responsible for reducing ribonucleotides to provide the deoxyribonucleotide precursors of DNA. In this study, we have tested the hypothesis that posttranscriptional regulation of ribonucleotide reductase R1 gene expression is controlled by a protein kinase C signal pathway. We show that mouse BALB/c 3T3 fibroblasts treated with the potent and highly specific protein kinase C inhibitor bisindolylmaleimide GF 109203X contain significantly reduced steady-state levels of R1 mRNA and protein. Message half-life studies demonstrate that this is due, at least in part, to a marked decrease in R1 message stability in cells treated with the protein kinase C inhibitor. Furthermore, the protein kinase C signal pathway appears to be specifically involved in this regulation since 8-bromo-cAMP, a modulator of the protein kinase A pathway, had no effect on R1 mRNA levels or stability properties. Cross-linking assays revealed that the binding activity of a R1 mRNA 3'-untranslated region binding protein (R1BP), which was previously shown to be involved in the regulation of R1 mRNA stability, was significantly elevated after treatment of the cells with GF 109203X, in a dose-dependent manner. However, treatment with 8-bromo-cAMP at concentrations up to 2.5 mM did not obviously affect the basic level of the R1BP-RNA interaction. These observations provide a better understanding of the biochemical signals that are critical for the cis-trans interaction-mediated posttranscriptional regulation of ribonucleotide reductase R1 gene expression.