Drosophila telomere transposons: genetically active elements in heterochromatin.

In Drosophila two non-LTR retrotransposons, HeT-A and TART, offer a novel experimental system for the study of heterochromatin. These elements, found only in heterochromatin, form Drosophila telomeres by repeated transposition onto chromosome ends. Their transposition yields arrays of repeats larger and more irregular than the repeats produced by telomerase; nevertheless, the transpositions are, in principle, equivalent to the telomere-building action of telomerase. The identification of the HeT-A promoter has given the first view of the molecular structure of a promoter active in heterochromatin. These telomere-specific elements are unusual in having a large amount of non-coding sequence. Like many other heterochromatic sequences, the HeT-A non-coding sequence has a repetitive organization strongly conserved within the species, although the sequence itself can undergo significant change between species (a typical example of concerted evolution). Such heterochromatic sequences could be important for the cell, perhaps as docking stations for essential proteins.