Institute for Evolution and Biodiversity, University of Muenster, Huefferstr. 1, D-48149 Muenster, Germany.
Mol Phylogenet Evol. 2011 Nov;61(2):321-32. doi: 10.1016/j.ympev.2011.06.023. Epub 2011 Jul 3.
Among eukaryotes two types of nuclear ribosomal DNA (nrDNA) organization have been observed. Either all components, i.e. the small ribosomal subunit, 5.8S, large ribosomal subunit, and 5S occur tandemly arranged or the 5S rDNA forms a separate cluster of its own. Generalizations based on data derived from just a few model organisms have led to a superimposition of structural and evolutionary traits to the entire plant kingdom asserting that plants generally possess separate arrays. This study reveals that plant nrDNA organization into separate arrays is not a distinctive feature, but rather assignable almost solely to seed plants. We show that early diverging land plants and presumably streptophyte algae share a co-localization of all rRNA genes within one repeat unit. This raises the possibility that the state of rDNA gene co-localization had occurred in their common ancestor. Separate rDNA arrays were identified for all basal seed plants and water ferns, implying at least two independent 5S rDNA transposition events during land plant evolution. Screening for 5S derived Cassandra transposable elements which might have played a role during the transposition events, indicated that this retrotransposon is absent in early diverging vascular plants including early fern lineages. Thus, Cassandra can be rejected as a primary mechanism for 5S rDNA transposition in water ferns. However, the evolution of Cassandra and other eukaryotic 5S derived elements might have been a side effect of the 5S rDNA cluster formation. Structural analysis of the intergenic spacers of the ribosomal clusters revealed that transposition events partially affect spacer regions and suggests a slightly different transcription regulation of 5S rDNA in early land plants. 5S rDNA upstream regulatory elements are highly divergent or absent from the LSU-5S spacers of most early divergent land plant lineages. Several putative scenarios and mechanisms involved in the concerted relocation of hundreds of 5S rRNA gene copies are discussed.
在真核生物中，已经观察到两种核核糖体 DNA（nrDNA）组织类型。要么所有组件，即小核糖体亚基、5.8S、大亚基和 5S 都串联排列，要么 5S rDNA 形成自己的独立簇。基于仅从少数模式生物获得的数据进行的概括导致将结构和进化特征叠加到整个植物界，断言植物通常具有独立的排列。本研究表明，植物 nrDNA 排列成独立的阵列不是一个独特的特征，而是几乎完全可归因于种子植物。我们表明，早期分化的陆地植物和推测的绿藻共享一个重复单元内所有 rRNA 基因的共定位。这增加了 rDNA 基因共定位状态可能发生在它们的共同祖先中的可能性。所有基础种子植物和水生蕨类植物都鉴定出单独的 rDNA 阵列，这意味着在陆地植物进化过程中至少发生了两次独立的 5S rDNA 转座事件。筛选可能在转座事件中发挥作用的 5S 衍生的 Cassandra 转座元件表明，这种逆转座子在包括早期蕨类植物谱系在内的早期分化的维管植物中缺失。因此，Cassandra 可以被拒绝为水生蕨类植物中 5S rDNA 转座的主要机制。然而，Cassandra 和其他真核 5S 衍生元件的进化可能是 5S rDNA 簇形成的副作用。核糖体簇的基因间间隔的结构分析表明，转座事件部分影响间隔区，并表明早期陆地植物中 5S rDNA 的转录调控略有不同。大多数早期分化的陆地植物谱系的 LSU-5S 间隔区中，5S rDNA 上游调控元件高度分化或缺失。讨论了涉及数百个 5S rRNA 基因拷贝协调重定位的几个假定方案和机制。
