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

每一个生物体都是由一组信息定义的，这些信息以基因的形式从它的祖先那里遗传下来，这些基因被一种叫做DNA的特定分子编码。因此，DNA包含了生物体在其环境中可能需要的所有东西的手册。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分子的稳定性和正确地将信息传递给下一代。为了确保子细胞获得所需的全部信息，每个细胞都要经历一个复杂的生长和分裂周期：细胞周期。这样的循环开始于一个只有一个遗传信息（DNA）副本的细胞，一旦某些参数得到满足，它就会完全复制。然后，只有在DNA完全复制并且完好无损的情况下，细胞分裂形成两个新生的子细胞，每个子细胞都有一个遗传信息的拷贝。这些细胞可以重新开始细胞周期，继续生长和分裂。一个重要的事实是，如果细胞的DNA受到某种程度的破坏，细胞将永远无法完成细胞周期。造成DNA损伤的原因有很多，包括辐射、阳光和环境中的化学物质；甚至我们呼吸的氧气也会对细胞中的DNA造成损害。一旦DNA受损，细胞会试图修复它。由于存在许多不同类型的损伤，因此存在许多不同类型的DNA修复机制，并且在某些情况下，不止一种修复途径可以处理特定类型的DNA损伤。重要的是要注意，并非所有的修复途径都同样准确，其中一些甚至可能在DNA密码中留下错误。因此，在每个特定时刻，调节哪一种机制能更好地修复病变是极其重要的。从对最危险的DNA损伤形式的修复的研究中可以清楚地看到上述调节类型的一个很好的例子；DNA双链断裂。有两种方法可以修复这种损伤。第一种（末端连接）是一种低精度的机制，它包括分子两端的直接重新连接。第二种是一种更为精确和复杂的称为同源重组的方法，它只在细胞已经复制了DNA的情况下才会发生，因此，DNA的第二个副本可以用作信息的供体。事实上，在DNA尚未复制时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，以确保同源重组仅在细胞复制了DNA后才被激活。我们的研究重点是理解这种相声的本质。我们用面包酵母酿酒酵母作为模型生物，发现它至少部分地依赖于一种叫做Sae2的蛋白质的调节，这种蛋白质只有在DNA被复制后才会被激活。Sae2的激活是DNA同源重组修复的必要条件。为了进一步了解Sae2是如何运作的，我们提出以下主要目标的建议：我们将确定Sae2如何调节两种修复途径（末端连接和同源重组）之间的平衡。2. 我们将确定帮助Sae2发挥其调节作用的其他因素。3. 我们将确定在细胞周期中控制同源重组的其他方法。4. 我们将把我们的工作扩展到研究减数分裂：一种特殊类型的细胞周期，它产生了有性生殖所必需的细胞。此外，我们实验室的另一名成员进行的研究已经确定了一种人类蛋白质，其功能似乎与酵母Sae2相同。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.