Cell Progression Through Meiosis: A Signal from Recombination to the First Division

减数分裂是真核生物在配子产生过程中染色体数量从二倍体减少到单倍体的特殊分裂过程，是一个复杂且高度保守的过程。可以把减数分裂的各个步骤想象成细胞内的发育途径。有丝分裂细胞获得进入减数分裂的信号，必须经过减数分裂前DNA合成、重组和突触、还原性分裂、等速分裂、单倍体细胞核按适当顺序包装等过程。一个有趣而重要的问题是，减数分裂的各个步骤如何相互沟通，以确保它们在正确的时间以正确的顺序发生。在萌芽的酵母，酿酒酵母中，一些问题的答案是已知的。确保事件正常发生的一个关键特征是有序的转录调控。进入减数分裂涉及有丝分裂抑制因子（RME1）的丢失和减数分裂激活因子（IME1）的表达。这导致了“早期减数分裂基因”的转录，其中包括减数分裂重组所需的基因。这些早期事件激活第二个转录调控因子NDT80，这是“中期减数分裂基因”的表达和第一次和第二次减数分裂所必需的。NDT80的激活也会导致“晚期减数分裂基因”的表达，尽管目前还不完全清楚这是NDT80激活中间基因的直接结果还是/和结果。减数分裂的第二层调控是减数分裂检查点。例如，执行减数分裂前复制的失败会导致细胞停滞。最近，人们意识到细胞也有一个评估减数分裂重组状态的检查点，这激发了人们的兴趣。在某些重组突变体（如dmc1）中，在减数分裂重组途径的中间点被阻断，细胞在第一次分裂前停滞。这种阻滞需要几个已知参与有丝分裂DNA损伤识别检查点的基因（例如，RAD17, MEC1）。有人提出，这种评估重组状态的检查点是减数分裂正常进程的一个关键特征，并且“dmc1检查点”也出现在野生型细胞中，因为中间产物在重组过程中正常产生。在重组过程中，染色体断裂和重新连接的高频率是一个细胞应该能够感知的过程，这是很有道理的。试图在重组完成之前分离染色体将是灾难性的。还有另一种方式，减数分裂重组与第一次分裂沟通，这种沟通发生在重组开始于野生型细胞。研究表明，减数分裂细胞能够识别重组已经开始；反应是将第一次分割延迟一段时间，这段时间相当于完成重组所需的时间。启动重组所需的四个基因（“EE”基因）的零突变导致更早的第一次分裂。这在直觉上是令人愉悦的；开始复杂的重组过程应该预示着减数分裂的下一个步骤，即第一次分裂，这似乎是非常合理的。这一信号传导过程似乎很复杂，因为不同EE基因的零突变对第一次分裂的时间有不同的影响。已经证明，该信号不是双链断裂的形成（重组起始中第一个容易观察到的DNA中间物）。这个起始信号的重要性可以通过注意到，在没有它的情况下，第一次分裂发生在同系物通常重组的时候。这表明，第一次分离装置可以准备就绪，并比正常情况下更早地发挥作用。这个实验室的数据表明，重组的开始可以防止这种过早的分裂。这个项目询问这种从重组到一阶分裂的新信号是如何工作的。它是否需要大多数起始基因，与信号是起始复合体形成的观点一致？突触复合体及其组成部分在传递信号中的作用是什么？重组起始的正常信号是否被检查点基因识别和传递，这些检查点基因也对后期发生的dmc1突变块做出反应？信号是否通过影响中央减数分裂调节剂NDT80的激活而起作用？最后，还有哪些基因参与了从重组到第一次分裂的信号？这项工作将定义细胞内信号传导过程是如何确保减数分裂的两个关键步骤在适当的时间发生的，这对减数分裂的正常进行至关重要。

Meiosis, the special division process in eukaryotes wherein chromosome number is reduced from diploid to haploid during the production of gametes, is complex and highly conserved. It is possible to think of the various steps of meiosis somewhat like an intracellular developmental pathway. Mitotic cells get a signal to enter meiosis and must go through premeiotic DNA synthesis, recombination and synapsis, reductional division, equational division, and packaging of the haploid nuclei in the proper order. An interesting and important question is how the various steps of meiosis communicate with each other to ensure that they occur at the right time and in the right sequence. In the budding yeast, Saccharomyces cerevisiae, answers to some of the questions are known. A key feature ensuring that events occur properly is ordered transcriptional regulation. Entry into meiosis involves the loss of a mitotic repressor (RME1) and expression of a meiotic activator (IME1). This results in the transcription of the "Early Meiotic Genes", which include the genes necessary for meiotic recombination. These early events activate a second transcriptional regulator, NDT80, which is required for expression of the "Middle Meiotic Genes" and the first and second meiotic division. NDT80 activation also results in the expression of "Late Meiotic Genes", though it is not yet completely clear whether this is direct or/and a consequence of NDT80 activation of the Middle genes. A second layer of regulation in meiosis is meiotic checkpoints. For example, a failure to perform premeiotic replication results in cell arrest. Recent interest has been stimulated by the realization that cells also have a checkpoint that assesses the state of meiotic recombination. In certain recombination mutants (e.g., dmc1) blocked at intermediate points in the meiotic recombination pathway, the cell arrests before the first division. This arrest requires several genes (e.g., RAD17, MEC1 ) known to be involved in mitotic DNA damage recognition checkpoints. It has been proposed that this checkpoint assessing the state of recombination is a key feature of normal progression through meiosis and that the "dmc1 checkpoint" also occurs in wildtype cells, as the intermediate is made normally during recombination. It makes good sense that the high frequency of breaking and rejoining chromosomes during recombination is a process which the cell should be able to sense. Attempts to segregate chromosomes before recombination was finished would be disastrous. There is yet another mode whereby meiotic recombination communicates with the first division, and that this communication occurs as recombination starts in wild type cells. It has been shown that meiotic cells are capable of recognizing that recombination has been started; the response is to delay the first division for a time equivalent to the time necessary to accomplish recombination. Null mutations in four genes required to initiate recombination ("EE" genes) result in a earlier first division. This is intuitively pleasing; it seems eminently reasonable that starting the complex process of recombination should signal the next meiotic step, the first division. It appears that this signaling process is complex since null mutations in different EE genes can have somewhat different effects on the timing of the first division. It has been shown that the signal is not the formation of double strand breaks (the first easily observed DNA intermediate in recombination initiation). The importance of this initiation signal is indicated by noting that, in its absence, the first division occurs at the time when homologs would normally be recombining. This indicates that the first division segregation apparatus can be ready and functional considerably earlier than it normally acts. Data from this laboratory indicates that the start of recombination prevents this premature division. This project asks how this novel signal from recombination to the first division works. Does it require the majority of the initiation genes, consistent with the idea that the signal is the formation of an initiation complex? What is the role of the synaptonemal complex and its component parts in sending the signal? Is the normal signal from recombination initiation recognized and communicated by the checkpoint genes that also respond to the dmc1 mutant block which occurs at later stages? Does the signal work by affecting activation of the central meiotic regulator NDT80? Finally, what are the other genes involved in this signal from recombination to the first division? This work will define how this intracellular signaling process, crucial for the proper progression through meiosis, functions to ensure that two critical steps in meiosis happen at the proper times.