Transcription analysis of the telomeric repeat-specific retrotransposons TRAS1 and SART1 of the silkworm Bombyx mori

The telomeres of the silkworm Bombyx mori consist of (TTAGG)n repeats and harbor a large number of sequence-specific non-LTR retrotransposons such as TRAS1 and SART1. In order to ascertain if TRAS1 and SART1 are transcribed in vivo and if there is a novel transcription mechanism peculiar to the sequence-specific retrotransposons, we studied their transcription. We detected transcripts of TRAS1 and SART1 by northern hybridization in many tissues and the BmN4 cell line of the silkworm. 5'-Rapid amplification of cDNA ends analysis showed that transcription of both elements was initiated precisely from their own 5'-ends and that most of their genomic copies contained these initiation sites. TRAS1 contained an internal promoter and positively regulating elements in the +1/+581 nucleotides in its 2432 bp 5'-untranslated region (UTR). We could not, however, detect any promoter activity in the SART1 5'-UTR. This difference may be related to the fact that only TRAS1 contained an initiator-like element at its 5'-end. Placing 1-52 units of the telomeric repeat (TTAGG)n upstream of TRAS1 reduced transcription 5-fold. The evidence suggests that most of the TRAS1 genomic copies within the telomeric repeats are weakly transcribed in vivo.