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

Abstract

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 (>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.The ends of eukaryotic chromosomes analyzed to date contain multiple repeats of very short, simple G-rich sequences (e.g., TTAGGG). These repeats are now considered the telomere sequences. There is evidence that the G-rich repeats can be added by telomerase, an enzyme that uses an RNA template to add copies of the telomere repeat to the chromosome end (5). Just internal to the telomere repeats, eukaryotic chromosomes have more complex sets of repeats (3,4,32) which are called telomere-associated repeats; however, their constant association with chromosome ends raises the possibility that telomere-associated sequences may be responsible for some of the functions that cytologists have suggested for the telomere. These possible functions include mediation of telomere-telomere and telomere-nuclear lamina interactions (14, 24). Thus, there is a possibility that telomere-associated sequences play a role in chromosome pairing at meiosis and/or in maintenance of the threedimensional organization of chromosomes within the nucleus.No simple telomerase-generated sequences have been found on Drosophila chromosomes. The failure to find simple repeats is not proof that they do not exist; however, it is possible that Drosophila species have lost this part of the ancestral telomere. On the other hand, Drosophila chromosomes do have larger, more complex telomere-associated sequences which may be evolutionarily rel...

Recurrent Deletion and Gene Presence/Absence Polymorphism: Telomere Dynamics Dominate Evolution at the Tip of 3L inDrosophila melanogasterandD. simulans

“…HeT-A elements are known to be telomere specific in their genomic distribution (Rubin 1978;Karpen and Spradling 1992;Levis et al 1993) and have been shown to ''heal'' broken chromosome ends through transposition (Beissmann et al 1990(Beissmann et al , 1992. Thus, these segregating deletions may represent independent terminal deletions that were healed by HeT-A transposition.…”

Similarities between long interspersed element-1 (LINE-1) reverse transcriptase and telomerase

“…For example, Drosophila lacks a formal telomerase and relies on three retrotransposons (HetA, TART, and TAHRE) to maintain its telomeres (61)(62)(63)(64)(65). Interestingly, TART and TAHRE have an apparent APE domain, suggesting that, unlike ENi L1 retrotransposition, this putative endonuclease may be important for telomere localization (66).…”

Telomere maintenance in Drosophila: Rapid transposon evolution at chromosome ends

“…29 The analysis of TART elements in different strains allowed the identification of three TART subfamilies. 32 Although HeT-A, TART and TAHRE are capable of healing broken chromosomes, [32][33][34][35][36] they have never been found in euchromatin. However, arrays of telomeric elements, decayed by numerous in/del mutations, have been found in centromeric and pericentromeric heterochromatin.…”