REGULATION OF RAR-BETA-2 MESSENGER-RNA EXPRESSION - EVIDENCE FOR AN INHIBITORY PEPTIDE ENCODED IN THE 5'-UNTRANSLATED REGION

Regulation of mRNA translation and stability plays an important role i n the control of gene expression during embryonic development, We have recently shown that the tissue-specific expression of the RAR beta 2 gene in mouse embryos is regulated at the translational level by short upstream open reading frames (uORFs) In the 5'-untranslated region (Z immer, A., A,M. Zimmer, and K. Reynolds. 1994. J. Cell Biol, 127:1111- 1119). To gain insight into the molecular mechanism, we have performed a systematic mutational analysis of the uORFs. Two series of construc ts were tested: in one series, each uORF was individually inactivated by introducing a point mutation in its start codon in the second serie s, all but one ORF were inactivated, Our results indicate that individ ual uORFs may have different functions, uORF4 seems to inhibit transla tion of the major ORF in heart and brain, while uORFs 2 and 5 appear t o be important for efficient translation in all tissues. To determine whether the polypeptide encoded by uORF4 or the act of translating it, is the significant event, we introduced point mutations to create sil ent mutations or amino acid substitutions in uORF4. Our results indica te that the uORF4 amino acid coding sequence is important for the inhi bitory effect on translation of the downstream major ORF.