TISSUE-SPECIFIC EXPRESSION OF THE RETINOIC ACID RECEPTOR-BETA-2 - REGULATION BY SHORT OPEN READING FRAMES IN THE 5'-NONCODING REGION

The 40-S subunit of eukaryotic ribosomes binds to the capped 5'end of mRNA and scans for the first AUG in a favorable sequence context to in itiate translation. Most eukaryotic mRNAs therefore have a short 5'unt ranslated region (5'-UTR) and no AUGs upstream of the translational st art site; features that seem to assure efficient translation. However, similar to 5-10% of all eukaryotic mRNAs, particularly those encoding for regulatory proteins, have complex leader sequences that seem to c ompromise translational initiation. The retinoic-acid-receptor-beta 2 (RAR beta 2) mRNA is such a transcript with a long (461 nucleotides) 5 '-UTR that contains five, partially overlapping, upstream open reading frames (uORFs) that precede the major ORE We have begun to investigat e the function of this complex 5'-UTR in transgenic mice, by introduci ng mutations in the start/stop codons of the uORFs in RAR beta 2-lacZ reporter constructs. When we compared the expression patterns of mutan t and wild-type constructs we found that these mutations affected expr ession of the downstream RAR beta 2-ORF, resulting in an altered regul ation of RAR beta 2-lacZ expression in heart and brain. Other tissues were unaffected. RNA analysis of adult tissues demonstrated that the u ORFs act at the level of translation; adult brains and hearts of trans genic mice carrying a construct with either the wild-type or a mutant UTR, had the same levels of mRNA, but only the mutant produced protein . Our study outlines an unexpected role for uORFs: control of tissue-s pecific and developmentally regulated gene expression.