The Y chromosome of Drosophila melanogaster contains a distinctive subclass of Het-A-related repeats.

The HeT-A element is a transposable element with an apparent role in the structure of the telomeres of Drosophila melanogaster chromosomes. HeT-A transposition is the earliest event detected in healing of broken ends; HeT-A is also found on telomeres of unbroken chromosomes. Sequences with homology to HeT-A are never detected in euchromatic regions; however, clusters of HeT-A-related sequences occur in nontelomeric regions of the heterochromatic Y chromosome. Analysis of two of these Y-associated clusters shows them to be significantly different in structure from telomeric HeT-A elements, although the regions of shared sequence have > 80% sequence identity in all cases. Telomeric HeT-A elements occur in chains, with the elements in the same orientation but variably truncated at their external ends and irregularly interspersed with unrelated sequences. In contrast, the nontelomeric Y elements are regular tandem repeats of parts of the HeT-A sequence joined to unrelated sequences which are not the same in the two clusters studied. The sequence structures suggest that the nontelomeric clusters on the Y chromosome do not arise by the same transposition mechanism that forms the telomeric clusters; instead the clusters on the Y may arise by a mechanism that is used more generally in the evolution of Y chromosomes. Although the telomeric and nontelomeric clusters appear to be formed differently, both are enriched in parts of the HeT-A sequence which may be important in the structure of heterochromatin.

HeT-A, a transposable element specifically involved in "healing" broken chromosome ends in Drosophila melanogaster.

Eight terminally deleted Drosophila melanogaster chromosomes have now been found to be "healed." In each case, the healed chromosome end had acquired sequence from the HeT DNA family, a complex family of repeated sequences found only in telomeric and pericentric heterochromatin. The sequences were apparently added by transposition events involving no sequence homology. We now report that the sequences transposed in healing these chromosomes identify a novel transposable element, HeT-A, which makes up a subset of the HeT DNA family. Addition of HeT-A elements to broken chromosome ends appears to be polar. The proximal junction between each element and the broken chromosome end is an oligo(A) tract beginning 54 nucleotides downstream from a conserved AATAAA sequence on the strand running 5' to 3' from the chromosome end. The distal (telomeric) ends of HeT-A elements are variably truncated; however, we have not yet been able to determine the extreme distal sequence of a complete element. Our analysis covers approximately 2,600 nucleotides of the HeT-A element, beginning with the oligo(A) tract at one end. Sequence homology is strong (greater than 75% between all elements studied). Sequence may be conserved for DNA structure rather than for protein coding; even the most recently transposed HeT-A elements lack significant open reading frames in the region studied. Instead, the elements exhibit conserved short-range sequence repeats and periodic long-range variation in base composition. These conserved features suggest that HeT-A elements, although transposable elements, may have a structural role in telomere organization or maintenance.