Cell cycle control of homologous recombination by the Sae2 protein in Saccharomyces cerevisiae

每个活着的有机体都是由一组信息定义的，这些信息是以基因的形式从其祖先那里继承下来的，这些基因是由一种名为DNA的特定分子编码的。因此，DNA构成了有机体在其环境中取得成功所需的所有东西的手册。由于其对生存的根本重要性，细胞已经发展出一系列机制来确保DNA分子的稳定性和信息的正确传递给下一代。为了确保子代细胞获得所需的所有信息，每个细胞都经历了一个复杂的生长和分裂周期：细胞周期。这样的循环始于只有一份遗传信息(DNA)的细胞，一旦满足某些参数，该基因信息就会完全复制。然后，只有在DNA完全复制且未受损的情况下，细胞才会分裂形成两个新生子细胞，每个细胞都有单一的遗传信息副本。然后，这些细胞可以重新启动细胞周期，继续生长和分裂。一个重要的事实是，如果DNA以某种方式受损，细胞将永远不会在细胞周期中前进。造成DNA损伤的原因有很多，包括辐射、阳光和环境化学物质；即使我们呼吸的氧气也会导致我们细胞中的DNA损伤。一旦DNA受损，细胞就会试图修复它。由于有许多不同类型的损伤，所以有许多不同类型的DNA修复机制，在某些情况下，多个修复途径可以处理特定类型的DNA损伤。重要的是要注意，并不是所有的修复途径都同样准确，其中一些甚至可能在DNA代码中留下错误。因此，调节哪种机制更好地修复每个给定时刻的损伤是极其重要的。对修复最危险的DNA损伤形式--DNA双链断裂--的研究表明，上述类型的监管是一个很好的例子。有两种方法可以修复这种损害。第一种(末端连接)是一种低精度的机制，包括分子两端的直接重新连接。第二种是更为准确和复杂的同源重组，只有当细胞已经复制了它的DNA时才会发生，因此，DNA的第二个副本可以用作信息的供体。事实上，在DNA尚未复制的情况下激活这一途径对细胞非常有害，并可能导致细胞死亡。这意味着细胞周期和DNA修复途径之间需要有串扰，以确保只有当细胞复制了他们的DNA时，同源重组才会被激活。我们的研究重点是了解这种相声的性质。使用面包师的酵母酿酒酵母作为模式生物，我们发现，这至少部分依赖于一种名为Sae2的蛋白质的调节，这种蛋白质只有在DNA复制后才会被激活。Sae2的激活是DNA同源重组修复所必需的。为了进一步了解Sae2是如何发挥作用的，我们提出了以下主要目标：1.我们将确定Sae2如何调节两条修复途径(末端连接和同源重组)之间的平衡。2.我们将确定有助于SAe2发挥其监管作用的其他因素。3.我们将确定在细胞周期中控制同源重组的其他方式。4.我们将把我们的工作扩展到减数分裂的研究上：减数分裂是一种特殊的细胞周期，产生有性繁殖所必需的细胞。此外，我们实验室的另一名成员进行的研究发现，一种人类蛋白质似乎具有与酵母Sae2相同的功能。因此，我们将与他合作研究人类细胞中类似的控制机制。

Each living organism is defined by a set of information that is inherited from its ancestors in the form of genes that are encoded by a specific kind of molecule called DNA. Therefore, the DNA comprises the handbook for all the things the organism may need to succeed in its environment. Because of its fundamental importance for survival, cells have developed a series of mechanisms to ensure both the stability of the DNA molecule and the proper transfer of the information to the next generation. In order to ensure that daughter cells get all the information they need, each cell undergoes a complex growth and division cycle: the cell cycle. Such a cycle starts with a cell that has just a single copy of the genetic information (DNA) that, once some parameters are fulfilled, becomes exactly duplicated. Then, and only if the DNA is completely duplicated and is undamaged, the cell divides to form two newborn daughter cells, each of them with a single copy of the genetic information. These cells can then re-start the cell cycle and continue growing and dividing. One important fact is that the cell will never progress through the cell cycle if its DNA is somehow damaged. There are many causes of DNA damage, which these include radiation, sunlight and environmental chemicals; and even the oxygen we breathe can result in damage to the DNA in our cells. Once the DNA is damaged, the cell will try to repair it. As there are many different kinds of damage, there are many different kinds of DNA repair mechanisms and, in some cases, more than one repair pathway can deal with a specific type of DNA damage. It is important to notice that not all the repair pathways are equally accurate and some of them may even leave errors in the DNA code. Therefore, the regulation of which mechanism is better to repair a lesion at each given moment is extremely important. A good example of the above types of regulation has become clear from research into the repair of the most dangerous form of DNA damage; the DNA double-strand break. There are two methods to repair such damage. The first one (end-joining) is a low accuracy mechanism that consists in direct rejoining of both ends of the molecule. The second one, a much more accurate and complex one called homologous recombination, only occurs when the cell has already duplicated its DNA and, therefore, the second copy of the DNA can be used as a donor of information. Activation of this pathway when the DNA has not been yet duplicated is, in fact, highly deleterious to the cell and can lead to its death. This means that there needs to be crosstalk between the cell cycle and the DNA repair pathways to ensure that homologous recombination is only activated once the cells have duplicated their DNA. Our research is focused on understanding the nature of this crosstalk. Using the baker's yeast Saccharomyces cerevisiae as a model organism, we have found that depends, at least partially, on the regulation of a protein called Sae2, that is activated only once the DNA has been copied. Activation of Sae2 is essential for DNA repair by homologous recombination. In order to further understand how Sae2 is acting, we present this proposal with the following major objectives: 1. We will determine how Sae2 regulates the balance between the two repair pathways (end-joining and homologous recombination). 2. We will identify other factors that help Sae2 in its regulatory roles. 3. We will identify other ways that homologous recombination is controlled during the cell cycle. 4. We will extend our work to study meiosis: the specialized type of cell cycle that produces the cells that are essential for sexual reproduction. In addition, research carried out by another member of our laboratory has identified a human protein that appears to function in the same way as yeast Sae2. Therefore, we will collaborate with him to study similar control mechanisms in human cells.