Full-length RNA transcribed from the human LINE-1 (Li) element Li Homo sapiens (LlHs) has a 900-nt, G+C-rich, 5'-untranslated region (UTR). The 5' UTR is followed by two long open reading frames, ORF1 and ORF2, which are separated from each other by an inter-ORF region of 33 nt that includes two or three in-frame stop codons. We examine here the mechanism(s) by which the translation of LlHs ORF1 and ORF2 is initiated. A stable hairpin structure (AG =-74.8 kcal/mol), inserted at nt 661 of the 5' UTR, caused a 3-to 8-fold decrease in the in vitro and in vivo translation ofeither a lacZ reporter gene for ORF1 or the ORF1 polypeptide product, p40, but translation of a lacZ reporter gene in ORF2 was increased. The results are compatible with a model for ORF1 translation initiation in which the majority of ribosomes scan from a point 5' of nt 661 but suggest that ORF2 is not translated by attached ribosomes that reinitiate after the termination of ORF1 translation. Our data are compatible with a model whereby the translation of LlHs ORF2 is initiated internally. The human LINE-1 (Li) element (Li Homo sapiens; LiHs) is the only known transposable element that is endogenous to the human genome. The structural features of LlHs (sche-matically presented in Fig.
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