HET-A, A RETROPOSON WITH A ROLE IN CHROMOSOME STRUCTURE

我们已经确定了一类新的DNA序列，这些序列只在 异染色质，但与简单重复的 以前被描述为异染色质特定序列的DNA。这 一类新的序列，HET DNA家族，来自果蝇 黑猩猩。多线染色体加上多条染色体 可用的重排和标记突变使D。 研究结构和结构的最好的模型系统 异染色质的功能；然而，我们认为这些结果 研究将对所有后生动物的染色体有深入的了解。 HET DNA的主要成分是一种名为HET-A的转座元件。 尽管HET-A元件与共享的序列相关联 在端粒和着丝粒周围异染色质之间(使 HET DNA的其他成分)，HET-A元件本身仅被发现 在端粒异染色质中。HET-A元素已添加到 D中已知的十个“治愈”末端缺失的断端。 黑猩猩。这些元素显然是通过换位添加的 不涉及序列同源性的事件，而是 与断裂的染色体连接表明是逆转座。如果这个 如果是真的，逆转座明显受到限制。-HET-A元素 与已知的转座元件不同之处在于具有2kb的保守区 短距离序列在一端重复(这一端总是 附着在断裂的染色体末端)。HET-A元素含量丰富 未断裂的染色体的端粒。HET-A元素的这些特征 这表明，虽然它们是转座元件，但它们可能有一个 端粒在端粒组织或维持中的结构作用。HET-A元素 与在端粒上发现的非常短的重复序列不同 有机体，但它们似乎类似于“端粒相关” 在后生动物染色体中发现的序列。因此，其他生物体也可能 有非“自私的DNA”的转座元件。 我们建议对HET-A和任何RNA编码的蛋白质进行表征 从元素转录而来。有理由认为换位 是可以诱导的。我们将努力确定在什么类型的细胞和什么下 条件RNA和蛋白质的表达是通向理解的一步 移位的诱导。我们马上就能找到HET-A元素 果蝇物种的进化保守特征分析 元素。我们提出了几个转基因实验来测试 关于HET-A和HET-A编码蛋白可能功能的结论 非编码重复片段。最后，我们将尝试使用 我们定义的序列是为了构建具有功能的微型果蝇- 染色体。这些染色体还将允许直接测试 结论。

We have identified a new class of DNA sequences that are found only in heterochromatin, yet' differ significantly from the tandemly repeated DNAs previously characterized as heterochromatin-specific sequences. This new class of sequences, the HeT DNA family, is from Drosophila melanogaster. The polytene chromosomes plus the many chromosomal rearrangements and marker mutations that are available make D. melanogaster the best model system for studying the structure and function of heterochromatin; however we believe that the results of these studies will give insights into the chromosomes of all metazoa. A major component of HeT DNA is a transposable element named HeT-A. Although HeT-A elements are associated with sequences that are shared between telomeric and pericentric heterochromatin (sequences that make up other components of HeT DNA), HeT-A elements themselves are found only in telomeric heterochromatin. HeT-A elements have been added to the broken ends of the ten "healed" terminal deletions now known in D. melanogaster. The elements have apparently been added by transposition events involving no sequence homology, instead the structure of the junction with the broken chromosome suggests retrotransposition. If this is true, the retrotransposition is remarkably restricted.- HeT-A elements differ from the known transposable elements in having >2 kb of conserved short-range sequence repeats on one end (the end that invariably is attached to the broken chromosome end). HeT-A elements are abundant on the telomeres of unbroken chromosomes. These features of HeT-A elements suggest that, although they are transposable elements, they may have a structural role in telomere organization or maintenance. HeT-A elements do not resemble the very short repeats found at the telomeres in other organisms but they appear to be similar to the "telomere-associated" sequences found in metazoan chromosomes. Thus other organisms may also have transposable elements that are not "selfish DNA". We propose to characterize the protein encoded by HeT-A and any RNAs transcribed from the element. There is reason to think that transposition is inducible. We will seek to determine in what cell types and under what conditions RNA and protein are expressed as a step toward understanding the induction of transposition. We will seek HeT-A elements in a second Drosophila species to analyze the evolutionarily conserved features of the element. We propose several transgenic experiments to test conclusions about possible functions of the protein encoded by HeT-A and of the non-coding repeated segment. Finally, we will attempt to use the sequences that we define to construct functional Drosophila mini- chromosomes. These chromosomes will also allow direct tests of conclusions.