Abstract
HeT-A was the first transposable element shown to have a bona fide role in chromosome structure, maintenance of telomeres in Drosophila melanogaster. HeT-A has hallmarks of non-long-terminal-repeat (non-LTR) retrotransposable elements but also has several unique features. We have now isolated HeT-A elements from Drosophila yakuba, showing that the retrotransposon mechanism of telomere maintenance predates the separation of D. melanogaster and D. yakuba (5-15 million years ago). HeT-A elements from the two species show significant sequence divergence, yet unusual features seen in HeT-Amel are conserved in HeT-Ayak. In both species, HeT-A elements are found in head-to-tail tandem arrays in telomeric heterochromatin. In both species, nearly half of the HeT-A sequence is noncoding and shows a distinctive imperfect repeat pattern of A-rich segments. Neither element encodes reverse transcriptase. The HeT-Amel promoter appears to be intermediate between the promoters of non-LTR and of LTR retrotransposons. The HeT-Ayak promoter shows similar features. HeT-Amel has a frameshift within the coding region. HeT-Ayak does not require a frameshift but shows conservation of the polypeptide sequence of the frameshifted product of D. melanogaster.
