LINE1 (also L1 and LINE-1) are transposable elements in the DNA of some organisms and belong to the group of Long interspersed nuclear elements (LINEs). L1 comprise approximately 17% of the human genome.
The majority of L1 in the human genome are inactive; however, some retained the ability to retrotranspose. Human L1 has been reported to have transferred to the genome of the gonorrhea bacteria.
A typical L1 element is approximately 6,000 base pairs long and consists of two non-overlapping open reading frames (ORF) which are flanked by UTR and target site duplications. In humans, ORF2 is thought to be translated by an unconventional termination/reinitiation mechanism, while mouse L1s contain an internal ribosome entry site (IRES) upstream of each ORF.
The 5' UTR of mouse L1s contain a variable number of GC-rich tandemly repeated monomers of around 200bp, followed by a short non-monomeric region.
Human 5’UTRs are ~900bp in length and do not contain repeated motifs. All families of human L1s harbor in their most 5’ extremity a binding motif for the transcription factor YY1. Younger families have also two binding sites for SOX-family transcription factors, and both YY1 and SOX sites were shown to be required for human L1 transcription initiation and activation.
The first ORF encode a 500 amino acid - 40kDa protein that lacks homology with any protein of known function. In vertebrates, it contains a conserved C-terminus domain and a highly variable coiled-coil N-terminus that mediates the formation of ORF1 trimetric complexes. ORF1 trimers have RNA-binding and nucleic acid chaperone activity that are necessary for retrotransposition.
- L1Base, a database of functional annotations & predictions of active LINE1 elements
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