INHIBITION OF CCAAT ENHANCER-BINDING PROTEIN ALPHA AND BETA TRANSLATION BY UPSTREAM OPEN READING FRAMES/

CCAAT/enhancer-binding protein (C/EBP) alpha is a bZIP transcription f actor whose expression is restricted to specific cell types. Analysis of C/EBP alpha mRNA and protein levels in various mammalian cells indi cates that expression of this gene is controlled both transcriptionall y and post-transcriptionally. We report here that C/EBP alpha translat ion is repressed in several cell lines by an evolutionarily conserved upstream open reading frame (uORF), which acts in cis to inhibit C/EBP alpha translation. Mutations that disrupt the uORF completely abolish ed translational repression of C/EBP alpha. The related c/ebp beta gen e also contains an uORF that suppresses translation. The length of the spacer sequence between the uORF terminator and the ORF initiator cod on (7 bases in all c/ebp alpha genes and 4 bases in c/ebp beta homolog s) is precisely conserved. The effects of insertions, deletions, and b ase substitutions in the C/EBP alpha spacer showed that both the lengt h and nucleotide sequence of the spacer are important for efficient tr anslational repression. Our data indicate that the uORFs regulate tran slation of full-length C/EBP alpha and C/EBP beta and do not play a ro le in generating truncated forms of these proteins, as has been sugges ted by start site multiplicity models.