Departamento de Genética e Biologia Evolutiva, Instituto de Biociências, Universidade de São Paulo, São Paulo, Brazil.
PLoS One. 2012;7(8):e44006. doi: 10.1371/journal.pone.0044006. Epub 2012 Aug 30.
Nucleoli, nuclear organelles in which ribosomal RNA is synthesized and processed, emerge from nucleolar organizers (NORs) located in distinct chromosomal regions. In polytene nuclei of dipterans, nucleoli of some species can be observed under light microscopy exhibiting distinctive morphology: Drosophila and chironomid species display well-formed nucleoli in contrast to the fragmented and dispersed nucleoli seen in sciarid flies. The available data show no apparent relationship between nucleolar morphology and location of NORs in Diptera. The regulation of rRNA transcription involves controlling both the transcription rate per gene as well as the proportion of rRNA genes adopting a proper chromatin structure for transcription, since active and inactive rRNA gene copies coexist in NORs. Transcription units organized in nucleosomes and those lacking canonical nucleosomes can be analyzed by the method termed psoralen gel retarding assay (PGRA), allowing inferences on the ratio of active to inactive rRNA gene copies. In this work, possible connections between chromosomal location of NORs and proportion of active rRNA genes were studied in Drosophila melanogaster, and in chironomid and sciarid species. The data suggested a link between location of NORs and proportion of active rRNA genes since the copy number showing nucleosomal organization predominates when NORs are located in the pericentric heterochromatin. The results presented in this work are in agreement with previous data on the chromatin structure of rRNA genes from distantly related eukaryotes, as assessed by the PGRA.
核仁是合成和加工核糖体 RNA 的核细胞器，它起源于位于不同染色体区域的核仁组织者 (NORs)。在双翅目昆虫的多线染色体核中，可以通过光学显微镜观察到一些物种的核仁，它们具有独特的形态：与丝角蝇目中看到的碎片化和弥散的核仁相比，果蝇和摇蚊属物种的核仁形态完整。现有数据表明，在双翅目昆虫中，核仁形态与 NORs 的位置之间没有明显的关系。rRNA 转录的调控涉及控制每个基因的转录速率以及 rRNA 基因采用适当染色质结构进行转录的比例，因为 NORs 中同时存在活性和非活性的 rRNA 基因拷贝。可以通过称为补骨脂素凝胶阻滞测定法 (PGRA) 的方法分析组织在核小体中的转录单位和缺乏典型核小体的转录单位，从而可以推断出活性和非活性 rRNA 基因拷贝的比例。在这项工作中，研究了 NORs 在染色体上的位置与活性 rRNA 基因比例之间的可能联系，研究对象包括黑腹果蝇、摇蚊属和丝角蝇目物种。数据表明，NORs 的位置与活性 rRNA 基因的比例之间存在联系，因为当 NORs 位于着丝粒异染色质中时，表现出核小体组织的拷贝数占主导地位。本工作的结果与通过 PGRA 评估的来自远缘真核生物的 rRNA 基因染色质结构的先前数据一致。
