Weis Benjamin L, Palm Denise, Missbach Sandra, Bohnsack Markus T, Schleiff Enrico
Department of Biosciences, Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt/Main, Germany.
Department of Biosciences, Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt/Main, Germany Institute for Molecular Biology, Georg-August University, 37073 Göttingen, Germany.
RNA. 2015 Mar;21(3):415-25. doi: 10.1261/rna.047563.114. Epub 2015 Jan 20.
Ribosome biogenesis is an essential process in all organisms. In eukaryotes, multiple ribosome biogenesis factors (RBFs) act in the processing of ribosomal (r)RNAs, assembly of ribosomal subunits and their export to the cytoplasm. We characterized two genes in Arabidopsis thaliana coding for orthologs of yeast BRX1, a protein involved in maturation of the large ribosomal subunit. Both atBRX1 proteins, encoded by AT3G15460 and AT1G52930, respectively, are mainly localized in the nucleolus and are ubiquitously expressed throughout plant development and in various tissues. Mutant plant lines for both factors show a delay in development and pointed leaves can be observed in the brx1-2 mutant, implying a link between ribosome biogenesis and plant development. In addition, the pre-rRNA processing is affected in both mutants. Analysis of the pre-rRNA intermediates revealed that early processing steps can occur either in the 5' external transcribed spacer (ETS) or internal transcribed spacer 1 (ITS1). Interestingly, we also find that in xrn2 mutants, early processing events can be bypassed and removal of the 5' ETS is initiated by cleavage at the P' processing site. While the pathways of pre-rRNA processing are comparable to those of yeast and mammalian cells, the balance between the two processing pathways is different in plants. Furthermore, plant-specific steps such as an additional processing site in the 5' ETS, likely post-transcriptional processing of the early cleavage sites and accumulation of a 5' extended 5.8S rRNA not observed in other eukaryotes can be detected.
核糖体生物发生是所有生物体中的一个基本过程。在真核生物中，多种核糖体生物发生因子（RBFs）参与核糖体（r）RNA的加工、核糖体亚基的组装及其向细胞质的输出。我们对拟南芥中编码酵母BRX1直系同源物的两个基因进行了表征，BRX1是一种参与大核糖体亚基成熟的蛋白质。分别由AT3G15460和AT1G52930编码的两种拟南芥BRX1蛋白主要定位于核仁，在植物发育过程中和各种组织中普遍表达。这两个因子的突变株系均表现出发育延迟，在brx1-2突变体中可观察到叶片变尖，这意味着核糖体生物发生与植物发育之间存在联系。此外，两个突变体中的前体rRNA加工均受到影响。对前体rRNA中间体的分析表明，早期加工步骤可发生在5'外部转录间隔区（ETS）或内部转录间隔区1（ITS1）。有趣的是，我们还发现，在xrn2突变体中，早期加工事件可以被绕过，5'ETS的去除是由P'加工位点的切割启动的。虽然前体rRNA加工途径与酵母和哺乳动物细胞的相似，但植物中这两种加工途径之间的平衡有所不同。此外，还可以检测到植物特有的步骤，如5'ETS中的一个额外加工位点、早期切割位点可能的转录后加工以及在其他真核生物中未观察到的5'延伸的5.8S rRNA的积累。
