Chromatin Structure in Drosophila Telomeres

Drosophila Chromosome Structure Group

Instead of the simple telomeric repeats found in other organisms, Drosophila uses non-LTR retrotransposons to elongate its chromosome ends. Targeted transposition creates terminal tandem arrays of retrotransposons with the oligo(A) tails facing the chromosome. One of these retrotransposon families, HeT-A, makes up the majority of elements in the terminal arrays, is found at virtually all chromosome ends, lacks a gene for reverse transcriptase, and yet transposes under laboratory conditions. In collaboration with Harald Biessmann, Ph.D. of the University of California, Irvine, the group is investigating the regulation of transcription of these telomere specific transposons.

In addition to the terminal retrotransposon array, telomeres also have subterminal repeats, termed telomere-associated sequence (TAS). While TAS repeats are found in all species examined, the sequence varies among species and even among chromosome ends within an individual. In Drosophila, TAS causes repression of neighboring reporter genes and appears to be heterochromatic, while expression of transgenes in terminal retrotransposon array resembles the level of expression of the same reporter in euchromatin.

In collaboration with Trevor Archer, Ph.D., head of the Chromatin & Gene Expression Group and Chief of the Laboratory of Molecular Carcinogenesis at NIEHS, the group is examining chromatin structure around telomeres using chromatin immunoprecipitation to identify the presence of specific heterochromatic proteins and modified histones, and Micrococcal nuclease and DNase I sensitivity to study nucleosome organization. The prediction is that chromatin structure will differ between silenced and suppressed genotypes. Expression of HeT-A, as determined by real-time RT PCR, will also indicate whether the terminal retrotransposons are responding to a change in silencing.

The repressed telomeric transgenes have also allowed monitoring of HeT-A movement to a chromosome end. The addition of HeT-A to the 2L tip occurs at a frequency of about 1% in a wild-type genetic background. A mutation has been identified on chromosome 2 that reduces HeT-A addition to less than 10-4 and another mutation, Telomere elongation (Tel) that greatly increases HeT-A addition onto chromosome ends.

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