PROMOTING IN TANDEM - THE PROMOTER FOR TELOMERE TRANSPOSON HET-A AND IMPLICATIONS FOR THE EVOLUTION OF RETROVIRAL LTRS

HeT-A elements are non-long terminal repeat (non-LTR) retrotransposons found in head-to-tail arrays on Drosophila chromosome ends, where the y form telomeres. We report that HeT-A promoter activity is located in the 3' end of the element, unlike the 5' location seen for other non- LTR retrotransposons. In HeT-A arrays the 3' sequence of one element d irects transcription of its downstream neighbor. Because the upstream promoter has the same sequence as the 3' end of the transcribed elemen t, the HeT-A promoter is effectively equivalent to a 5' LTR in both st ructure and function. Retroviruses and LTR retrotransposons have their promoters and transcription initiation sites in their 5' LTRs. Thus H eT-A appears to have the structure of an evolutionary intermediate bet ween non-LTR and LTR retrotransposons.