RNA methylation, surveillance and the plant immune response

DNA是由四种不同类型的“碱基”组成的长链：A、C、G和T。DNA的信息内容主要存在于这些信息沿其长度沿着出现的顺序中。遗传密码被复制到一种称为RNA的相关分子中，RNA是该密码的信使。这种信使RNA（mRNA）从细胞核中移出，并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mRNA中时，RNA本身的碱基会发生特定的变化。在植物和动物的mRNA中最常见的修饰是在腺苷上添加一个小的化学“标签”，以在称为甲基化的过程中制造m6 A。m6 A通常添加在mRNA的末端附近，靠近翻译成蛋白质末端的位点，但我们知道有些信息含有几个m6 A位点，而另一些则不含。15年前，我们证明了这种修饰是植物和酵母正常发育所必需的，随后其他研究小组也证明了动物也是如此。我们设计成具有低水平m6 A的植物发育不良，具有特定的叶和发育缺陷，并组成性地激活病原体防御反应。其原因是m6 A的存在可以影响mRNA的加工方式，何时降解以及它作为蛋白质合成模板的次数。然而，m6 A实现这一点的机制仍然知之甚少。甲基化被细胞核和细胞质m6 A结合“YTH”蛋白“读取”。在细胞核中，YTH蛋白可以参与告诉细胞在哪里停止将DNA复制成RNA，而在细胞质中，不同的YTH蛋白告诉细胞是否降解，翻译或储存mRNA。在植物和动物中，细胞质YTH蛋白与UPF 1（UP-Frameshift-1）相互作用，UPF 1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。UPF 1的突变也组成性地激活免疫反应，并产生具有类似叶子缺陷的发育不良的植物。通过在“抑制筛选”中突变数千粒种子，并进行有针对性的遗传杂交，我们已经表明，防御信号传导途径的失活恢复了低m6 A植物的正常植株高度和叶片形状。类似的杂交也能恢复UPF 1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m6 A转化回A。FTO、m6 A“writers”和YTH“readers”都与几种严重的人类疾病有关，因此在分子水平上了解m6 A的功能对全球制药公司和研究人员来说非常重要。使用植物作为模型系统，我们已经确定了一组酶的作家，共同采取行动，把“标签”的mRNA，我们已经表明，标签通常被放置在mRNA的末端附近。随后发现这些酶和mRNA甲基化的特征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实验简单得多，可以揭示可能也适用于人类的基本原理。具有低m6 A的植物对某些细菌病原体更具抗性，因此理解书写、阅读的功能以及与UPF 1“质量控制”的整合将为用于开发更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m6 A调节某些mRNA如何被UPF 1依赖的监视途径降解或抑制的方式。这是重要的，因为它可能是一个保守的过程，在真核生物中的m6 A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.