Post-transcriptional regulation of the GAP-43 gene by specific sequences in the 3' untranslated region of the mRNA.

We have shown previously that GAP-43 gene expression during neuronal differentiation is controlled by selective changes in mRNA stability. This process was found to depend on highly conserved sequences in the 3' untranslated region (3' UTR) of the mRNA. To map the sequences in the GAP-43 3' UTR that mediate this post-transcriptional event, we generated specific 3' UTR deletion mutants and chimeras with the beta-globin gene and measured their half-lives in transfected PC12 cells. Our results indicate that there are two distinct instability-conferring elements localized at the 5' and 3' ends of the GAP-43 3' UTR. Of these destabilizing elements, only the one at the 3' end is required for the stabilization of the mRNA in response to treatment with the phorbol ester TPA. This 3' UTR element consists of highly conserved uridine-rich sequences and contains specific recognition sites for two neural-specific GAP-43 mRNA-binding proteins. Analysis of the levels of mRNA and protein derived from various 3' UTR deletion mutants indicated that all mutants were translated effectively and that differences in gene expression in response to TPA were attributable to changes in GAP-43 mRNA stability. In addition, the phorbol ester was found to affect the binding 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 GAP-43 mRNA-binding proteins are expressed primarily in neural cells, we propose that these factors may be involved in the post-transcriptional regulation of GAP-43 gene expression during neuronal differentiation.