Identification of the earliest land plant based on the complete structure of chloroplast genome and RNA editing

绿色植物发展史上最重要的问题之一是哪一种是最早的陆地植物。我们已经确定了金鱼藻的叶绿体基因组的完整结构，它曾被认为是最早的陆地植物。ML树基于已知的叶绿体基因组中常见的51个蛋白质基因的10,854个氨基酸残基，揭示了苔藓植物是单系植物，具有很高的统计置信度，并形成了最早的分支，与其他陆地植物的主要谱系有所不同。这棵树表明，金鱼藻在苔藓植物中是基础的。另一个重要的问题是通过分析一个有望成为陆地植物直系祖先的轮藻的完整叶绿体结构来解决另一个重要问题。与其他叶绿体相比，金鱼藻叶绿体中发现了更多的RNA编辑位点。它们含有436个U到C的转化位点，没有观察到任何种子植物。这些编辑转换了一半以上的功能基因，并创建了tRNA的反密码子和剪接所需的序列。几乎所有与基因突变互补的RNA编辑都需要在早期的陆地植物中进行，因为RNA编辑在藻类中不存在，但几乎在所有陆地植物中都能看到。与编辑后的序列互补的序列被称为远端顺式识别元件(DGRE)，在同一个RNA中被观察到。在碱基配对不匹配的情况下，过早的转录本可以形成双链结构。这种错配可以被识别，并通过编辑酶进行纠正。这种假想的RNA编辑机制将在体外得到验证。

One of the most important problem remaining in the phylogeny of green plants is which is the earliest land plant. We have determined the complete structure of the chloroplast genome from hornwort which had been expected as the earliest land plant. ML tree, based on 10,854 amino acid sites from 51 protein genes which are commonly found in the known chloroplast genomes, revealed that bryophytes are monophyletic with high statistical confidence and form the earliest branch, which diverged from all the other main lineages of land plants. The tree shows that the hornwort is basal in bryophytes.Another important problem, which is the direct ancestor of land plants, is in progress by analyzing the complete chloroplast structure from a charales which is expected as the direct ancestor of land plants.Much more RNA editing sites have been found in the hornwort chloroplast than those in the other chloroplasts. They contained 436 sites of U to C conversion which is not observed any seed plants. These editing converts more than half the genes functional, and creates the anticodon of a tRNA and the sequence required for splicing. Almost all the RNA editing complementing the mutation in the genes would be required in early land plants, because RNA editing is not seen in the algae but seen in almost all land plants.The sequence, named distant cis-recognition element (DGRE), complementary to the edited sequence has been observed within the same RNA. The premature transcript could form double stranded structure where there is a mismatch in base pairing. The mismatch could be recognized, and corrected by editing enzyme. This hypothetical mechanism for RNA editing will be proven in vitro.