Drosophila telomeres: two transposable elements with important roles in chromosomes

Telomeres in Drosophila melanogaster are composed of multiple copies of two retrotransposable elements, HeT-A and TART instead of the short DNA repeats generated by telomerase in most organisms. Transpositions of HeT-A and TART yield arrays of repeats larger and more irregular than the repeats produced by telomerase; nevertheless, these transpositions are, in principle, equivalent to the telomere-building action of telomerase. Both telomerase and transposition of HeT-A and TART extend chromosomes by RNA-templated addition of specific sequences. We have proposed that HeT-A has evolved from genes encoding telomerase components. Although both HeT-A and TART share some novel features, TART probably has a different origin from HeT-A. HeT-A and TART are clearly identifiable as non-long terminal repeat (non-LTR) retrotransposons. Both telomere elements transpose only to the ends of chromosomes (apparently to any chromosome end in D. melanogaster) and each contains a large segment of untranslated sequence. HeT-A and TART are the first examples of transposable elements with a clear role in chromosome structure. This has interesting implications for the evolution of both chromosomes and transposable elements. The finding also raises the possibility that other transposable elements with bona fide roles in the cell will be detected, not only in Drosophila, but also in other organisms.