ELEMENTS IN THE MURINE C-MOS MESSENGER-RNA 5'-UNTRANSLATED REGION REPRESS TRANSLATION OF DOWNSTREAM CODING SEQUENCES

Murine c-mos transcripts isolated from testes have 5'-untranslated reg ions (5'UTRs) of similar to 300 nucleotides with a series of four over lapping open reading frames (ORFs) upstream of the AUG codon that init iates the Mos ORF. Ovarian c-mos transcripts have shorter 5'UTRs (70-8 0 nucleotides) and contain only 1-2 of the upstream ORFs (uORFs). To t est whether these 5'UTRs affect translational efficiency, we have cons tructed plasmids for the expression of chimeric transcripts with a mos -derived 5'UTR fused to the Escherichia coli P-galactosidase coding re gion. Translational efficiency has been evaluated by measuring beta-ga lactosidase activity in NIH3T3 cells transiently transfected with thes e plasmids and with plasmids where various mutations have been introdu ced into the 5'UTR. We show that the 5'UTR characteristic of testis-sp ecific c-mos mRNA strongly represses translation relative to the trans lation of transcripts that contain a 5'UTR derived from beta-globin mR NA, and this is mainly due to the four uORFs. Each of the four upstrea m AUG triplets can be recognized as a start site for translation, and no single uAUG dominates the repressive effect. The uORFs repress tran slation by a mechanism that is not affected by the amino acid sequence in the COOH-terminal region of the uORF-encoded peptides. The very sh ort uORF (AUGUGA) present in ovary-specific transcripts does not repre ss translation, Staining of testis sections from transgenic mice carry ing chimeric beta-galactosidase transgene constructs, which contain a mos 5'UTR with or without the uATGs, suggests that the uORFs can drama tically change the pattern of expression in spermatogenic cells.