TRANSLATION START SITE MULTIPLICITY OF THE CCAAT ENHANCER-BINDING PROTEIN-ALPHA MESSENGER-RNA IS DICTATED BY A SMALL 5' OPEN READING FRAME

The CCAAT/enhancer binding proteins (C/EBP) alpha and beta of the bZIP family of transcription factors each occur as multiple forms due to t ranslation initiation at different in-frame AUG codons from the same m essenger RNA. The C/EBP alpha mRNAs of chicken, rat and Xenopus all co ntain a small 5' open reading frame (5'ORF) whose size (18 nucleotides ) and distance (seven nucleotides) to the C/EBP alpha cistron has been conserved in vertebrate evolution. The present studies shows that the small 5'ORF is crucial to the leaky scanning mechanism of ribosomes c ausing a fraction of them to ignore the first C/EBP alpha AUG codon an d to start at internal AUGs. Our data challenge the view that translat ional start site multiplicity is mainly governed by the sequence conte xt of the potential initiation codons. Western analysis showed that th e two major chicken C/EBP alpha translation products, the full-length cC/EBP alpha-42 which acts a trans-activator in liver and the N-termin ally truncated cC/EBP alpha-29 which lacks transcription activation po tential, occur in a fixed ratio which is similar in different expressi ng tissues, like liver, lung and small intestine. The presence of a si milar, thusfar unnoticed, small ORF 5' to the major initiation codon o f C/EBP beta mRNA suggests that start site multiplicity from this mRNA may be governed by the same mechanism.