Functional Consequences Of The Plant Epitranscriptome

DNA由四种不同类型的“碱基”的长链组成：A、C、G和T。DNA的信息量主要按照这些碱基在其长度上出现的顺序排列。遗传密码被复制到一个相关的分子中，称为RNA，它是这个密码的信使。这种信使RNA(信使RNA)从细胞核中移出，被称为核糖体的细胞机械用来作为构建蛋白质的模板。RNA由几乎相同的碱基A、C和G组成，但U取代了T。当基因复制到信使核糖核酸中后，信使核糖核酸本身的碱基可以发生特定的变化。在植物和动物的mRNA中，最常见的修饰是在腺苷中添加一个小的化学“标签”，在一个被称为甲基化的过程中生成M6A。M6A在mRNA中的出现频率约为0.1-0.2%。这相当于每个典型消息平均一到两次，但我们知道一些消息包含几个m6A站点，而另一些消息则不包含。M6A“标签”的存在不会改变翻译过程中合并的氨基酸，它的功能在30多年里一直是个谜。十年前，我们证明了植物和酵母的正常发育程序需要这种修饰，随后在动物身上也证明了这一点。其原因是，m6A的存在会影响mRNA的处理方式、何时降解以及它被用作蛋白质合成模板的次数。然而，m6A实现这一点的机制仍未得到很好的理解。甲基化过程本身是动态的。人类脂肪量和肥胖相关基因FTO已被证明编码一种蛋白质，可以去除“标签”并将m6A转化回A。FTO与几种严重的人类疾病有关，因此在分子水平上了解m6A的功能对世界各地的制药公司和研究人员来说都很重要。使用植物作为模型系统，我们已经确定了共同作用于mRNA上的一组酶，并且我们已经证明，标签通常放置在mRNA的末端附近。这些酶和mRNA甲基化的特征随后被发现在植物和动物之间高度保守。因此，在植物中进行的实验比在哺乳动物中进行的简单得多，可以揭示可能也适用于人类的基本原理。我们设计的具有低水平m6A的植物很小，很弱，并且有特定的发育缺陷。通过随机突变这些植物的数千颗种子，我们发现一些植物生长正常，尽管它们的m6A水平仍然很低。恢复正常生长的突变之一是编码一种蛋白质的基因，这种蛋白质构成核糖体的一部分。这种核糖体蛋白是重新开始或继续翻译具有特定结构的mRNAs所必需的，无论是在植物中还是在某些人类病毒中。该项目的目的是了解m6A调控核糖体翻译某些mRNAs的方式。这一点很重要，因为它可能是一个保守的过程，也是人类过程中m6A功能的基础。我们还将识别使我们的低m6A植物正常生长的其他基因的突变，我们预计类似的基因功能也将存在于人类中，并且从我们的植物模型系统中获得的见解将继续为研究领域提供信息。

DNA is comprised of long chains of "bases" of four different types: A, C, G and T. The information content of DNA resides primarily in the order in which these occur along its length. The genetic code is copied into a related molecule called RNA that is the messenger of this code. This messenger RNA (mRNA) moves out from the cell nucleus and is used by cellular machinery called ribosomes as a template on which to build proteins. RNA is comprised of almost the same bases, A, C, and G, but U replaces T. After a gene has been copied into mRNA, specific changes can be made to the bases of the mRNA itself. The most common modification within mRNA of both plants and animals is the addition of a small chemical "tag" to adenosines to make m6A in a process referred to as methylation. The frequency of m6A in mRNA is about 0.1-0.2%. This would correspond to an average of roughly once or twice per typical message, but we know that some messages contain several m6A sites whilst others contain none. The presence of the m6A "tag" does not change which amino acids are incorporated during translation and its function remained a mystery for more than 30 years. Ten years ago we showed that this modification was needed for normal developmental programmes in plants and yeast and other groups subsequently showed that this was also the case in animals. The reason for this is that the presence of m6A can affect how a mRNA is processed, when it is degraded and how many times it is used as a template for protein synthesis. However, the mechanisms by which m6A achieves this are still poorly understood.The methylation process is itself dynamic. The human fat mass and obesity associated gene, FTO, has been shown to encode a protein that can remove the "tag" and convert m6A back to A. FTO is linked to several serious human diseases, so understanding the function of m6A at a molecular level has become important for drug companies and researchers worldwide. Using plants as a model system, we have identified the group of enzymes that act together to put the "tag" on the mRNA and we have shown that the tag is usually placed near the end of mRNA. These enzymes and features of mRNA methylation were subsequently found to be highly conserved between plants and animals. Thus, experiments in plants that are much simpler to perform than in mammals can reveal fundamental principles that are likely to operate in humans also.Plants that we have engineered to have low levels of m6A are small, weak and have specific developmental defects. By randomly mutating thousands of seeds from these plants, we found some plants that grew normally, even though they still had very low m6A levels. One of the mutations that restores normal growth is in a gene that encodes a protein that forms part of the ribosome. This ribosomal protein is needed to restart or continue translation of mRNAs with certain structures, both in plants and in some human viruses. The aim of this project is to understand the way in which m6A regulates how some mRNAs are translated by ribosomes. This is important because it is likely a conserved process that underlies m6A function in human processes too. We will also identify the mutations in other genes that allow our low m6A plants to grow normally, we anticipate that similar gene functions will also be present in human and that insights gained from our plant model system will continue to inform the research field.