Báez Mariana, Souza Gustavo, Guerra Marcelo
Universidade Federal de Pernambuco, Departamento de Botânica, Laboratório de Citogenética e Evolução de Plantas, Recife, PE, Brazil.
Genet Mol Biol. 2020 Mar 6;43(1):e20180194. doi: 10.1590/1678-4685-GMB-2018-0194. eCollection 2020.
35S ribosomal DNA (rDNA) sites are the regions where the ribosomal genes 18S, 5.8S and 25S, responsible for the formation of the nucleoli, are found. The fact that rDNA sites have non-random distribution on chromosomes suggests that their positions may influence their transcription. To identify if the preferentially transcribed rDNA sites occupy specific position, six species (nine cytotypes) of the genus Nothoscordum were analyzed using two different techniques to impregnate the nucleolar organizer regions (NORs) with silver nitrate. Both techniques strongly stained NORs, but one of them also stained the proximal region of all chromosomes, suggesting the existence of another group of argentophilic proteins in this region. In species with rDNA sites in acrocentric and metacentric chromosomes, sites located on the short arms of the acrocentric chromosomes were preferentially activated. On the other hand, in species with rDNA sites restricted to the short arms of the acrocentrics, all of them were activated, whereas in those species with sites restricted to the terminal region of metacentric chromosomes, the frequency of active sites was always lower than expected. This indicate that, at least in Nothoscordum, the transcription of an rDNA site is influenced by its chromosomal position, and may explain, at least partially, the strongly non-random distribution of these sites in plant and animal chromosomes.
35S核糖体DNA（rDNA）位点是发现负责核仁形成的核糖体基因18S、5.8S和25S的区域。rDNA位点在染色体上具有非随机分布这一事实表明它们的位置可能会影响其转录。为了确定优先转录的rDNA位点是否占据特定位置，使用两种不同的技术用硝酸银浸渍核仁组织区（NORs），对六线蒜属的六个物种（九种细胞型）进行了分析。两种技术都能强烈地将NORs染色，但其中一种技术还能将所有染色体的近端区域染色，这表明该区域存在另一组嗜银蛋白。在近端着丝粒染色体和中着丝粒染色体上有rDNA位点的物种中，位于近端着丝粒染色体短臂上的位点优先被激活。另一方面，在rDNA位点仅限于近端着丝粒染色体短臂的物种中，所有这些位点都被激活，而在那些位点仅限于中着丝粒染色体末端区域的物种中，活性位点的频率总是低于预期。这表明，至少在六线蒜属中，rDNA位点的转录受其染色体位置的影响，并且这可能至少部分地解释了这些位点在植物和动物染色体中强烈的非随机分布。
