TELOMERE STRUCTURE IN DROSOPHILA

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Summary of Work: Telomeres are structures on the ends of linear chromosomes that are required for genome stability. Drosophila telomeres lack the simple tandem repeats found at telomeres in most species. Instead, Drosophila ends are maintained by tip specific transposition of a non-LTR retrotransposon family, HeT-A. Adjacent to the terminal transposon array is a telomere associated satellite (TAS) that resembles subtelomeric sequences in other species. The TAS repeats may inactivate genes placed in their vicinity, and form associations with other telomeres. In Drosophila, reporter genes placed in TAS are repressed and variegate. In many cases a reporter gene in TAS is repressed when the other telomeres are wild type, but less repressed when any other telomere is disrupted by an insertion into TAS or a deletion of TAS. We propose that the telomeres associate with each other, as has been found in yeast, and that disruption of one telomere signals the retrotransposons on other telomeres to transcribe HeT-A, resulting in relaxed chromatin around the transgene. Mutations of zeste, a pairing-dependent transcription factor, disrupt this interaction, supporting the hypothesis of physical interactions among telomeres affecting gene expression. The induced transcription may promote HeT-A retrotransposition and, telomere elongation. In a related project, 54 trans-acting suppressers of telomeric repression in Drosophila have been isolated and are being characterized. Similar modifiers of centromeric repression were used previously to identify heterochromatin proteins, but none of these has an effect on telomeric repression. Suppressers of telomeric silencing map to all the major chromosomes; several are homozygous lethal, suggesting they perform a vital function. They suppress the activity of genes inserted into various telomeres, but not genes inserted into centromeric heterochromatin. They also have no effect on long-term developmental silencing, suggesting that they represent a new class of chromosomal protein.