HeT DNA: a family of mosaic repeated sequences specific for heterochromatin in Drosophila melanogaster.

HeT DNA is a complex family of repeated DNA found only in pericentric and telomeric heterochromatin. In contrast to other DNA families that have been specifically associated with heterochromatin, HeT DNA is not principally a family of tandemly repeated elements. Much of the HeT DNA family appears to be a mosaic of several different classes of large sequence elements arranged in a scrambled array; however, some elements of the family can be found in tandem repeats. In spite of the variable order of the different elements in HeT DNA, the sequence homology between different members of each class of element is extremely high, suggesting that the members are evolving in a concerted fashion. Sequence analysis suggests that some elements in the HeT family may make up a novel family of heterochromatin-specific transposable elements and that the mosaic organization of the elements may be produced by retroposition and other mechanisms involved in the transposition of mobile elements. We suggest that such mechanisms may be a general feature for the maintenance of chromosome structure.