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目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看作是细胞内的发育途径。 有丝分裂细胞获得进入减数分裂的信号，并且必须经历减数分裂前DNA合成、重组和联会、减数分裂、均等分裂和以适当顺序包装单倍体核。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和正确的顺序发生。在芽殖酵母中，酿酒酵母，一些问题的答案是已知的。确保事件正确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阻遏物（RME 1）的丢失和减数分裂激活物（IME 1）的表达。 这导致“早期减数分裂基因”的转录，其包括减数分裂重组所必需的基因。这些早期事件激活了第二个转录调节因子NDT 80，它是“中期减数分裂基因”以及第一次和第二次减数分裂表达所必需的。NDT 80激活也导致“晚期减数分裂基因”的表达，尽管还不完全清楚这是直接的还是/和中间基因的NDT 80激活的结果。 减数分裂的第二层调控是减数分裂检查点。例如，未能进行减数分裂前复制导致细胞停滞。最近的兴趣已经刺激的认识，细胞也有一个检查点，评估减数分裂重组的状态。在某些重组突变体（例如，dmc 1）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止。这种抑制需要几个基因（例如，RAD 17，MEC 1）已知参与有丝分裂DNA损伤识别检查点。有人提出，这个评估重组状态的检查点是减数分裂正常进展的关键特征，并且“dmc 1检查点”也发生在野生型细胞中，因为中间体在重组过程中正常产生。重组过程中染色体断裂和重新连接的高频率是细胞应该能够感知的过程，这是很有道理的。 在重组完成之前试图分离染色体将是灾难性的。 还有另一种模式，减数分裂重组与第一次分裂沟通，这种沟通发生在野生型细胞中重组开始时。已经表明，减数分裂细胞能够识别重组已经开始;其反应是将第一次分裂延迟一段时间，该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）中的突变导致更早的第一次分裂。这是直观的愉悦;似乎非常合理的是，开始复杂的重组过程应该是下一个减数分裂步骤，即第一次分裂的信号。这个信号传导过程似乎很复杂，因为不同EE基因中的无效突变可能对第一次分裂的时间产生不同的影响。已经表明，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中间体）。这一起始信号的重要性是通过注意到，在其缺失的情况下，第一次分裂发生在同源物通常重组的时候来表示的。 这表明，第一分割分离装置可以比其正常动作早得多地准备好并起作用。来自这个实验室的数据表明，重组的开始阻止了这种过早的分裂。 这个项目询问这种从重组到第一次分裂的新信号是如何工作的。它是否需要大多数起始基因，与信号是起始复合物形成的想法一致？联会复合体及其组成部分在发送信号中的作用是什么？重组启动的正常信号是否被检查点基因识别和传递，而这些基因也对发生在后期的dmc 1突变阻断有反应？信号是否通过影响中央减数分裂调节因子NDT 80的激活而起作用？最后，从重组到第一次分裂的信号中还涉及哪些其他基因？这项工作将定义这种细胞内信号传导过程，对减数分裂的正确进展至关重要，如何发挥作用以确保减数分裂中的两个关键步骤在适当的时间发生。

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.