Chromatin Structure in Drosophila Telomeres

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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. 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. At least some of the transcription through these transgenes initiates in the upstream retrotransposon in the terminal array. 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, although we have found that the heterochromatin protein, HP1, associates with transgenic markers in the retrotransposon array, and the level of HP1 correlates with the level of transcriptional activity. Defects in, or deletions of, one of these TAS arrays at the tip of chromosome 2L increase transgene activity at all other measured telomeres. By extension we infer that retrotransposon transcription is increased in the presence of shortened telomere, which would be a mechanism for maintaining a steady-state telomere length.