RNA methylation, surveillance and the plant immune response

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，被细胞机器用作构建蛋白质的模板。RNA由几乎相同的碱基A、C和G组成，但U取代了T。当基因被复制到信使核糖核酸中时，RNA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通常被添加到mRNA末端附近，靠近翻译成蛋白质末端的位置，但我们知道一些消息包含几个M6A位点，而另一些消息则不包含。15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。我们设计的m6A水平较低的植物发育迟缓，具有特定的叶片和发育缺陷，并结构性地激活病原体防御反应。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实现这一目标的机制仍然知之甚少。甲基化是由核和细胞质中结合m6A的“YTH”蛋白“读取”的。在细胞核中，YTH蛋白可能参与告诉细胞在哪里停止将DNA复制到RNA中，而在细胞质中，不同的YTH蛋白告诉细胞是降解、翻译还是储存一种信使。在植物和动物中，细胞质中的YTH蛋白与UPF1(Up-FrameShift-1)相互作用，UPF1是一种通过确定是否达到正确的翻译末端来参与“质量控制”的蛋白质。UPF1的突变也会结构性地激活免疫反应，使发育迟缓的植物具有类似的叶缺陷。通过在“抑制子筛选”中突变数千个种子，并进行有针对性的遗传杂交，我们已经表明，在低m6A植物中，防御信号通路的失活可以恢复正常的植物身材和叶形。类似的杂交还可以恢复UPF1突变体的正常生长和叶片缺陷。甲基化过程本身是动态的。人类脂肪质量和肥胖相关基因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、m6A“写入者”和YTH“读取者”都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都很重要。利用植物作为一个模型系统，我们已经确定了一组共同作用的酶编写者，它们将“标签”放在mRNA上，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。低m6A的植物对某些细菌病原体具有更强的抗性，因此了解UPF1的写入、读取和整合的功能将为开发高产抗病作物的标记辅助育种计划提供信息。该项目的目的是了解m6A如何调控一些mRNAs如何被UPF1依赖的监视途径降解或翻译抑制。这一点很重要，因为它可能是一个保守的过程，是真核生物m6A功能的基础。

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 as a template on which to build proteins. RNA is comprised of almost the same bases, A, C, and G, but U replaces T. As a gene is being copied into mRNA, specific changes can be made to the bases of the RNA 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 m6A is usually added near the end of the mRNA, close to the site were translation in to protein ends, but we know that some messages contain several m6A sites whilst others contain none. Fifteen 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. Plants that we engineered to have low levels of m6A are stunted, have specific leaf and developmental defects and constitutively activate pathogen defence responses. 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 is "read" by nuclear and cytoplasmic m6A binding "YTH" proteins. In the nucleus, YTH proteins can be involved in telling the cell where to stop copying the DNA into RNA, and in the cytoplasm, different YTH proteins tell the cell whether to degrade, translate or store an mRNA. In both plants and animals, cytoplasmic YTH proteins interact with UPF1 (UP-Frameshift-1), a protein involved in "quality control" by determining if the correct end to translation has been reached. Mutation of UPF1 also constitutively activates the immune response and gives stunted plants with similar leaf defects. By mutating thousands of seeds in a "suppressor screen", and by carrying out targeted genetic crosses, we have shown that inactivation of defence signalling pathways restores normal plant stature and leaf shape in low m6A plants. Similar crosses also restore normal growth and leaf defects to UPF1 mutants.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, m6A "writers" and YTH "readers" are all 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 enzyme writers 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 with low m6A are more resistant to certain bacterial pathogens, thus understanding the function of writing, reading and integration with UPF1 "quality control" will inform marker assisted breeding programmes for developing higher yielding disease resistant crops. The aim of this project is to understand the way in which m6A regulates how some mRNAs are degraded or translationally suppressed by the UPF1-dependent surveillance pathway. This is important because it is likely a conserved process that underlies m6A function across eukaryotes.