Chromatin Structure in Drosophila Telomeres

Environmental Health Sciences

Investigators

Abstract

Telomeres are nucleoprotein structures at chromosome ends that are required to completely replicate the linear DNA and to distinguish the natural chromosome end from a double strand chromosome break for purposes of DNA repair. In Drosophila, chromosome ends are maintained by the targeted transposition of three retrotransposons, HeT-A, TAHRE and TART. A transgene inserted in the telomere between the subterminal telomere associated sequence (TAS) and the terminal retrotransposon array, or within TAS, is repressed and variegates. This variegation, termed telomeric position effect (TPE), appears to be due to an interaction of repression induced by TAS and activation initiated by transcription of the retrotransposons. Previous studies have identified genetic conditions in which TPE occurs and conditions in which it is suppressed. The purpose of this project is to identify changes in chromatin structure associated active and suppressed TPE. Nucleosome arrays are monitored by Micrococcal nuclease and DNase 1 digestion, and protein content assayed by chromatin immunoprecipitation. To date, no differences in the nucleosome array have been identified between TAS and the retrotransposon array, but protein content varies between these DNA domains. TAS preferentially binds chromatin proteins associated with heterochromatin, while the retrotransposons preferentially bind proteins associated with active chromatin. [unreadable] A transgene inserted into the telomere between the subterminal telomere associated sequence (TAS) and the terminal retrotransposon array is repressed and variegates. This variegation, termed telomeric position effect, TPE, appears to be due to an interaction of repression induced by TAS and activation initiated by HeT-A transcription. A telomeric transgene thus provides an assay for this interaction. These transgenes provide a means to investigate the control of HeT-A transcription and transposition, and thus telomere elongation. We have found that when the transgene is in or near TAS, transgene activity is repressed, and expression varies with changes in TAS sequences on the homologous telomere, as well as at other telomeres. Defects in, or deletions of, one of these TAS arrays increase transgene activity, and by extension we infer retrotransposon transcription.