Mobile genetic elements have been reported in prokaryotes, plants, yeast and Drosophila. The only transposon-like sequences reported for mammalian organisms are closely related to retroviruses, although undoubtedly other transposon families exist within the mammalian genome. Although mobile genetic elements can only be identified as such if their mobility can be demonstrated in existing populations, transposon and transposon-like elements share several common biochemical and structural features. Here we demonstrate that a repetitive human sequence has many of the diagnostic features of transposable elements. This 2.3-kilobase (kb) transposon-like element contains two flanking long terminal repeat (LTR)-like 350-base pair (bp) repetitive sequences, each of which begins with the sequence 5' TG... and ends with ...CA 3'. The transposon-like element is bounded by 5-bp direct repeats. Discrete-length polyadenylated transcripts from HeLa cells are homologous to the transposon-like element. Members of this transposon-like family are found in extrachromosomal circular DNA molecules.
The transcriptional activity of a human transposon-like family of repeats, called the THE-1 family, has been studied in cell culture and in human tissue. Both strands of THE-1 are present in several discrete length poly A plus RNAs. Primer extension studies and the structures of cDNA clones show that these THE-1 transcripts are usually the product of other transcription units. The THE-1 LTR provides the polyadenylation processing site for two transcripts, which result from upstream non THE-1 promoters. Yet another transcript, containing an internal THE-1 element in the probable sense orientation, is greatly enriched in a polysomal size fraction.
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