Human LINE-1 DNA elements (L1Hs) belong to a recently recognized, distinctive class of transposable elements ubiquitous in eukaryotes. Called class II or non-LTR, or polyadenylated retrotransposons, these elements have no long terminal repeats, one or more open reading frames, one of which predicts a polypeptide similar to known reverse transcriptases, and an A-rich 3' end on the strand with the open reading frames. Models describing the mechanism of transposition of these elements have been proposed but none has yet been demonstrated. All the models require the transcription and translation of the elements. Several years ago we demonstrated specific and full length L1Hs transcripts in human teratocarcinoma cells. We have now demonstrated, using appropriately constructed vectors and transient transfection, that 1) L1Hs contains internal transcriptional regulatory signals sufficient to promote transcription from the 5' end of an L1Hs element; 2) the first 100 bp from the 5' end contain essential regulatory sequences, and other significant regulatory sequences occur between bp 100 and 600 from the 5' end - the remaining 300 bp preceding the start of ORFI appear to be unimportant for transcription; 3) transcription in 2 human teratocarcinoma cell lines is approximately as efficient as that directed by the SV40 early promoter; 4) transcription from L1Hs sequences is much less efficient in HeLa cells, CV1 (monkey cells) and 3T3 (mouse cells). Using an antiserum prepared against the putative product of the L1Hs ORFI we have detected a polypeptide of the appropriate size in the cytoplasmic fraction of human cells. Thus, both transcription and translation of L1Hs occur, consistent with the proposed models. Moreover, it is possible that these functions, as well as transposition, are most active in early embryonic cells. The Drosophila melanogaster class II retrotransposon jockey has been shown to occur, in active form, in the distantly related species, D. funebris. This suggests the horizontal transfer of jockey within the Drosophila genus.
