title: The dicistronic RNA from the mouse LINE-1 retrotransposon contains an internal ribosome entry site upstream of each ORF: implications for retrotransposition.
abstract: Most eukaryotic mRNAs are monocistronic and translated by cap-dependent initiation. LINE-1 RNA is exceptional because it is naturally dicistronic, encoding two proteins essential for retrotransposition, ORF1p and ORF2p. Here, we show that sequences upstream of ORF1 and ORF2 in mouse L1 function as internal ribosome entry sites (IRESes). Deletion analysis of the ORF1 IRES indicates that RNA structure is critical for its function. Conversely, the ORF2 IRES localizes to 53 nt near the 3' end of ORF1, and appears to depend upon sequence rather than structure. The 40 nt intergenic region (IGR) is not essential for ORF2 IRES function or retrotransposition. Because of strong cis-preference for both proteins during L1 retrotransposition, correct stoichiometry of the two proteins can only be achieved post-transcriptionally. Although the precise stoichiometry is unknown, the retrotransposition intermediate likely contains hundreds of ORF1ps for every ORF2p, together with one L1 RNA. IRES-mediated translation initiation is a well-established mechanism of message-specific regulation, hence, unique mechanisms for the recognition and control of these two IRESes in the L1 RNA could explain differences in translational efficiency of ORF1 and ORF2. In addition, translational regulation may provide an additional layer of control on L1 retrotransposition efficiency, thereby protecting the integrity of the genome.
authors.affiliation: Cell and Developmental Biology, University of Colorado School of Medicine, 12801 E. 17th Avenue, Aurora, CO 80010, USA.
authors.forename: Patrick Wai-Lun
authors.initials: PW
authors.lastname: Li
authors.forename: Jinfang
authors.initials: J
authors.lastname: Li
authors.forename: Stephanie L
authors.initials: SL
authors.lastname: Timmerman
authors.forename: Les A
authors.initials: LA
authors.lastname: Krushel
authors.forename: Sandra L
authors.initials: SL
authors.lastname: Martin
mesh_terms: D000818:Animals; D002460:Cell Line; D020084:Long Interspersed Nucleotide Elements; D051379:Mice; D016366:Open Reading Frames; D014176:Protein Biosynthesis; D011506:Proteins / Q000235:genetics; D012333:RNA, Messenger / Q000737:chemistry*; D038621:Regulatory Sequences, Ribonucleic Acid; D012270:Ribosomes / Q000378:metabolism; D017384:Sequence Deletion
chemical_list: D011506:Proteins; D012333:RNA, Messenger; D038621:Regulatory Sequences, Ribonucleic Acid