The Trans Effect of Non-Functional L1 Loci on Retrotransposition of Functional L1 Element

Long Interspersed Element 1 (LINE-1, L1) is an autonomous non-LTR retroelement that is currently active in human and mammalian genomes. A full-length LINE-1 contains a 5' UTR, two open reading frames (ORF1 and ORF2) and a 3' UTR. Translation of L1 mRNA generates two proteins, ORF1 and ORF2, both of which are required for L1 retrotransposition. ORF1 and ORF2 proteins associate with their parental mRNA through a process known as cis preference to form RNA/protein complexes required for retrotransposition. L1 has accumulated to about 500,000 copies distributed throughout the genome, of which several thousands are full-length and about 100 are functional, capable of retrotransposition. Consequently, functional and non-functional L1 elements co-exist in mammalian genomes. Many of the full-length L1 loci have accumulated stop codons within their ORF1 sequence. These loci have the potential to produce truncated ORF1p with a yet unknown impact on retrotransposition.;Here we demonstrate that full-length ORF1p monomers generated from different expression plasmids can form heterocomplexes in mammalian cells. This trans association suggests that truncated ORF1 proteins may also interact in trans. By testing the trans effect of the full-length and truncated ORF1p on L1 retrotransposition in mammalian cells, we found that human and mouse ORF1 proteins have differential effects on retrotransposition of their respective L1 elements in human and mouse cells. This effect requires an N-terminus and coiled-coil domain of ORF1p. We demonstrate that a genetic disruption of the leucine zipper motif in the coil-coil domain of human ORF1p abolishes the suppressive effect on L1 retrotransposition. We also generated L1 elements containing mutations resulting in stop codons at specific positions identified by bioinformatic analysis of L1 loci residing in the human and mouse genomes. We found that some of the L1 loci containing stop codons express truncated ORF1 proteins and suppress retrotranspsoition of a functional L1 element. Taken together these findings suggest that L1 retrotransposition may be influenced by coexpression of defective L1 loci and that these L1 loci may reduce accumulation of de novo L1 integration events.