Burkholder G D, Latimer L J, Lee J S
Department of Anatomy, College of Medicine, University of Saskatchewan, Saskatoon, Canada.
Chromosoma. 1991 Oct;101(1):11-8. doi: 10.1007/BF00360681.
Purine.pyrimidine (pur.pyr) DNA tracts are prevalent in eukaryotic genomes. They can adopt a triplex conformation in vitro under conditions that may exist in vivo, suggesting that triplex (H-) DNA may exist naturally in chromosomes. To explore this possibility and gain insight concerning potential functions, the distribution of triplex DNA was studied in fixed polytene chromosomes of Chironomus tentans and Drosophila melanogaster by indirect immunofluorescence microscopy using an anti-triplex DNA monoclonal antibody (Jel 318). Chromosomes stained with this antibody exhibited immunopositive regions corresponding to condensed chromatin bands; interbands were less immunofluorescent. These results imply that there is more triplex DNA in bands than in interbands. In Chironomus, nucleolar organizer regions and Balbiani rings were immunonegative, indicating that triplex DNA is not present in decondensed, transcriptionally active chromatin. A few specific bands in both Chironomus and Drosophila were intensely immunofluorescent. In Drosophila, one such region was 81F on chromosome 3R. Competition during staining with exogenously added sequences corresponding to a constituent 1.672 g/cm3 satellite DNA in region 81F failed to abolish the immunofluorescence, suggesting that the satellite DNA does not fortuitously react with Jel 318 and implying that unidentified pur.pyr sequences forming triplex DNA are also present at this location. Region 81F exhibits ectopic pairing with nonrelated chromosome regions that have also proven to be intensely immunopositive; this suggests that the formation of triplex DNA between common, shared pur.pyr sequences in these otherwise nonhomologous bands might account for the ectopic pairing phenomenon. Together with our previous results, these data are consistent with the hypothesis that triplex DNA may play a role in chromosome organization by participating in regional chromatin condensation.
嘌呤 - 嘧啶（pur.pyr）DNA片段在真核生物基因组中普遍存在。在可能存在于体内的条件下，它们在体外可形成三链体构象，这表明三链体（H-）DNA可能天然存在于染色体中。为了探究这种可能性并深入了解其潜在功能，我们使用抗三链体DNA单克隆抗体（Jel 318），通过间接免疫荧光显微镜技术研究了摇蚊和黑腹果蝇固定多线染色体中三链体DNA的分布。用该抗体染色的染色体显示出与浓缩染色质带相对应的免疫阳性区域；带间的免疫荧光较弱。这些结果表明带中三链体DNA比带间更多。在摇蚊中，核仁组织区和巴尔比亚尼环呈免疫阴性，表明三链体DNA不存在于解聚的、转录活跃的染色质中。摇蚊和果蝇中的一些特定带强烈免疫荧光。在果蝇中，一个这样的区域位于3R染色体上的81F处。用与81F区域中一种组成性1.672 g/cm³卫星DNA对应的外源添加序列进行染色时的竞争未能消除免疫荧光，这表明卫星DNA并非偶然与Jel 318反应，意味着在该位置也存在形成三链体DNA的未鉴定的pur.pyr序列。81F区域与其他已被证明强烈免疫阳性的非相关染色体区域表现出异位配对；这表明在这些原本非同源的带中共同的、共享的pur.pyr序列之间形成三链体DNA可能解释了异位配对现象。与我们之前的结果一起，这些数据与三链体DNA可能通过参与区域染色质浓缩在染色体组织中发挥作用的假设一致。
