ALTERNATIVE LEADER SEQUENCES IN INSULIN-LIKE GROWTH-FACTOR-I MESSENGER-RNAS MODULATE TRANSLATIONAL EFFICIENCY AND ENCODE MULTIPLE SIGNAL PEPTIDES

Rat insulin-like growth factor I (IGF-I) mRNAs contain multiple 5'-unt ranslated regions due to the use of leader exons transcribed from seve ral transcription initiation sites and to alternative splicing within leader exon 1. Synthetic RNAs with 5'-ends corresponding to the use of exon 1 transcription initiation sites were translated in vitro into p repro-IGF-I peptides initiated at a Met-48 codon in exon 1 or a Met-22 codon in exon 3, and RNAs with a 5'-end corresponding to the major ex on 2 transcription start site were translated into a prepro-IGF-l pept ide initiated at a Met-32 codon in, exon 2. All forms of prepro-IGF-I were processed by canine pancreatic microsomes, suggesting that all th ese prepeptides function as signal peptides. The translational efficie ncy of IGF-I RNAs was inversely proportional to the length of the 5'-u ntranslated region. Mutation of the first of three upstream AUG codons in exon 1, which potentially initiates a 14-amino acid open reading f rame, did not affect prepro-IGF-l translation. The other two AUG codon s are immediately followed by stop codons. The absence of both upstrea m AUG codons in a completely spliced exon 1-derived RNA enhanced the i n vitro and in vivo translatability of this RNA as compared with the f ull-length RNA. Mutation of the downstream initiation codon in particu lar increased translational efficiency in vitro and in intact cells, s uggesting that an inefficient reinitiation event at the Met-48 codon c ontributes to the poorer translation of IGF-I mRNAs in which these ups tream AUGUGA motifs occur. We conclude that IGF-I mRNAs potentially en code multiple forms of preprolGF and that specific differences in thei r 5'-untranslated regions provide a molecular basis for translational control of IGF-I biosynthesis.