Locke J, Howard L T, Aippersbach N, Podemski L, Hodgetts R B
Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada.
Chromosoma. 1999 Nov;108(6):356-66. doi: 10.1007/s004120050387.
The banded portion of chromosome 4 (the "dot" chromosome) in Drosophila melanogaster displays some properties of beta-heterochromatin, which is normally found within the centric domain of the chromosomes. The nature and distribution of repetitive elements on chromosome 4 could play a role in the establishment of this unusual chromatin configuration. We describe here one such element: a short, interspersed repetitive sequence named DINE-1. Determination of a consensus sequence for the element reveals that there are two conserved regions (A and B) separated by a highly variable spacer. The conserved sequences are approximately 400 bp long but degenerate at both ends, opening the possibility that a yet-to-be-discovered mother element may be present in the genome. DINE-1 bears few of the properties of the mammalian short interspersed elements (SINEs) to which it bears a superficial resemblance in size. It does not appear to be the product of reverse transcription and lacks any polymerase III promoter consensus. The elements are not flanked by target site duplications and their termini lack direct or inverted repeats, suggesting that they themselves are not transposable. Our analysis of cosmid clones from chromosome 4, and elsewhere in the genome, revealed that the euchromatic locations of DINE-1 are almost exclusively confined to chromosome 4. In situ hybridization of a DINE-1 probe to polytene chromosomes confirmed the preferential distribution along 4, in addition to its presence in the centric heterochromatin. This unusual genomic distribution of bias toward chromosome 4 is also seen in the sibling species, D. simulans, whose dot chromosomes exhibit poorly resolved polytene bands and lack crossing over during meiosis like those of D. melanogaster. However, the dot chromosome of D. virilis, which exhibits a well-defined banded structure on polytene chromosomes and can cross over, has only a single, discrete site of DINE-1 element hybridization. The presence of DINE-1 within these regions suggests a role in the heterochromatic nature of chromosome 4 in D. melanogaster and supports the contention that repeats accumulate in regions of diminished crossing over.
果蝇黑腹果蝇4号染色体的带状部分（“点状”染色体）表现出一些β-异染色质的特性，β-异染色质通常存在于染色体的着丝粒区域内。4号染色体上重复元件的性质和分布可能在这种异常染色质构型的形成中起作用。我们在此描述一种这样的元件：一种名为DINE-1的短散在重复序列。对该元件的共有序列的测定表明，有两个保守区域（A和B）被一个高度可变的间隔区隔开。保守序列长度约为400 bp，但两端退化，这使得基因组中可能存在尚未发现的母元件。DINE-1几乎不具备哺乳动物短散在元件（SINEs）的特性，尽管它在大小上与SINEs有表面上的相似之处。它似乎不是逆转录的产物，也缺乏任何聚合酶III启动子共有序列。这些元件两侧没有靶位点重复，其末端也没有直接或反向重复，这表明它们本身不可转座。我们对来自4号染色体及基因组其他部位的黏粒克隆的分析表明，DINE-1在常染色质中的位置几乎完全局限于4号染色体。用DINE-1探针与多线染色体进行原位杂交证实，除了在着丝粒异染色质中存在外，它还优先分布在4号染色体上。在同胞物种拟果蝇中也观察到这种偏向于4号染色体的异常基因组分布，其点状染色体表现出分辨率差的多线带，并且在减数分裂过程中像黑腹果蝇一样缺乏交叉。然而，在多线染色体上表现出明确带状结构且能发生交叉的粗壮果蝇的点状染色体上，只有一个单一的、离散的DINE-1元件杂交位点。这些区域中DINE-1的存在表明它在黑腹果蝇4号染色体的异染色质性质中起作用，并支持重复序列在交叉减少区域积累的观点。
