POSTTRANSCRIPTIONAL REGULATION OF THE GAP-43 GENE BY SPECIFIC SEQUENCES IN THE 3'-UNTRANSLATED REGION OF THE MESSENGER-RNA

We have shown previously that GAP-43 gene expression during neuronal d ifferentiation is controlled by selective changes in mRNA stability. T his process was found to depend on highly conserved sequences in the 3 ' untranslated region (3' UTR) of the mRNA. To map the sequences in th e GAP-43 3' UTR that mediate this post-transcriptional event, we gener ated specific 3' UTR deletion mutants and chimeras with the beta-globi n gene and measured their half-lives in transfected PC12 cells. Our re sults indicate that there are two distinct instability-conferring elem ents localized at the 5' and 3' ends of the GAP-43 3' UTR. Of these de stabilizing elements, only the one at the 3' end is required for the s tabilization of the mRNA in response to treatment with the phorbol est er TPA. This 3' UTR element consists of highly conserved uridine-rich sequences and contains specific recognition sites for two neural-speci fic GAP-43 mRNA-binding proteins, Analysis of the levels of mRNA and p rotein derived from various 3' UTR deletion mutants indicated that all mutants were translated effectively and that differences in gene expr ession in response to TPA were attributable to changes in GAP-43 mRNA stability, In addition, the phorbol ester was found to affect the bind ing of specific RNA-binding proteins to the 3' UTR of the GAP-43 mRNA. Given that, like the GAP-43 mRNA, its degradation machinery and the G AP-43 mRNA-binding proteins are expressed primarily in neural cells, w e propose that these factors may be involved in the posttranscriptiona l regulation of GAP-43 gene expression during neuronal differentiation .