L1 (LINE-1) is a long interspersed repetitive sequence derived from a retrotransposon. Transfection studies using the CAT gene as a reporter demonstrated that the first 155bp in the human L1 sequence contains an element(s) responsible for the promoter activity in HeLa cells. The transcription was shown to initiate at the first nucleotide of the L1 sequence in the transgene. Three prominent nuclear protein binding sites were found in the 5′ region of the Li sequence by DNasel footprint analysis. One of the binding sites, designated as site A located at +3 to +26, was shown to be essential for the L1 transcription because the mutation at the site A caused almost complete loss of the promoter activity. A sequence AAGATGGCC at +11 to +19 in the site A was defined as a target core element for the protein binding. The site A-binding protein (designated TFL1-A) was found in various types of cells including an embryonic teratocarcinoma cell line. These results indicate that an internal short element located at the very 5' terminal of L1 sequence and the nuclear factor binding to the element play a crucial role in the transcription of human L1.
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