Telomere Structure In Drosophila

在果蝇中，染色体末端是通过两个反转录转座子HET-A和START的定向转座来维持的。在野生型中，转座的频率足以平衡由于不完整的DNA复制而导致的染色体末端的逐渐缩短。已知突变会显著增加或减少HET-A在染色体末端的添加频率，这表明这一过程是在基因控制下进行的。我们已经鉴定了一个这样的突变，我们正在使用位置信息来克隆该基因。插入端粒相关序列(TAS)的w+转基因被抑制并杂化。这种差异似乎是由于TAS诱导的抑制和HET-A启动的激活的相互作用所致。W+转基因为HET-A的转录提供了一种检测方法。因此，这些转基因可能为研究HET-A转录和转座以及端粒延长的机制提供了一种手段。我们发现，w+的活性随着同源端粒和其他端粒上TAS序列的变化而变化。这些TAS阵列中的一个的缺陷或缺失增加了w+的活性，进而增加了HET-A转录。这可能提供了一种在一个或多个端粒变短时增加转位的方法。为了研究HET-A的激活机制，我们收集了影响w+活性的辐射诱导突变和插入突变。虽然这些突变影响了几个不同端粒上的w+活性，但它们对非端粒位置的基因沉默没有影响。对这些突变的分析正在进行中。作图实验表明，2号染色体上的所有突变都位于左侧端粒或其附近。这些突变中的许多是纯合子致命性的，并且不能补充紧邻2L端粒的基因突变。

In Drosophila, chromosome ends are maintained by the targeted transposition of two retrotransposons, HeT-A and TART. In the wild type, the frequency of transposition is sufficient to balance the gradual shortening of chromosome ends due to incomplete DAN replication. Mutations are known that drastically increase or decrease the frequency of HeT-A addition to a chromosome end, suggesting that this process in under genetic control. We have characterized one such mutation, and we are using positional information to clone the gene. A w+ transgene inserted into the telomere associated sequence (TAS) immediately proximal to the terminal HeT-A/TART array is repressed and variegates. This variegation appears to be due to an interaction of repression induced by TAS and activation initiated by HeT-A. The w+ transgene provides an assay for HeT-A transcription. Thus, these transgenes may provide a means to investigate mechanisms of HeT-A transcription and transpostion, and telomere elongation. We have found that w+ activity 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 w+ activity, and by extension HeT-A transcription. This may provide a means to increase transposition when one or more telomeres become short. To investigate the mechanism of HeT-A activation, we have collected radiation-induced mutations and insertional mutations that affect w+ activity. While these mutations affect w+ activity on several different telomeres, they have no effect on gene silencing at non-telomeric positions. Analysis of these mutations is ongoing. Mapping experiments indicate that all of the mutations on chromosome 2 reside at or near the left telomere. Many of these mutations are homozygous lethal and fail to complement mutations in the gene that is immediately adjacent to the telomere at 2L.