LINE-1 - A HUMAN TRANSPOSABLE ELEMENT

Among the 10(5) LINE-1 sequences (L1Hs) in the human genome are one or more 6-kb segments that are active retrotransposons. Expression of th ese retrotransposons appears to be favored in cells of germ line origi n, as well as in some other tumor cells of epithelial origin. In such cells, the product of the first L1Hs open reading frame (ORF), a prote in called p40, is detectable; p40 has no apparent similarity to gag pr oteins, but contains a leucine zipper region which may be responsible for the occurrence of p40 multimers. Transcription of L1Hs initiates a t residue 1 although the transcriptional regulatory regions are downst ream in the first 670 bp of the 5' untranslated region; deletion of a YY1-binding site in the first 20 bp reduces transcription by fivefold. Translation of the second ORF, which encodes reverse transcriptase, i s independent of the translation of the frame encoding p40.