Abstract
Full-length RNA transcribed from the human LINE-1 (L1) element L1 Homo sapiens (L1Hs) 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 L1Hs ORF1 and ORF2 is initiated. A stable hairpin structure (delta G = -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 of either 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 L1Hs ORF2 is initiated internally.