Mechanisms Regulating the Activation of Early Meiosis-Specific Genes

作为对饥饿的反应，出芽的酵母，即酿酒酵母，停止生长和分裂，进入一个称为产孢的过程，导致单倍体孢子的产生。孢子对恶劣的环境条件具有抵抗力，使细胞能够在长时间的饥饿或干旱中存活。在重新喂食后，孢子可以萌发为单倍体细胞，然后进行交配过程。因此，在恶劣条件下生存和有性繁殖都需要产孢。产孢过程需要一组不同于细胞活跃生长和分裂时所表达的基因。这些孢子特异性基因可以根据其表达的时间划分为家族。早期基因对于孢子形成过程的开始是必要的。这些基因在生长过程中被蛋白质Ume6、Sin3和Rpd3关闭。饥饿时，IME1基因被诱导，IME1蛋白与Ume6结合，开启其他早期基因。其中之一是IME2。当Ime2蛋白积累时，它完全开启了早期基因。IME1基因在饥饿反应中被激活，但仅在具有完全功能的线粒体和能够呼吸的细胞中被激活。线粒体缺陷或呼吸抑制会阻断IME1转录和整个产孢过程。本提案的目的是确定呼吸能力如何调节IME1基因的表达，以及Ime2如何驱动孢子形成所需的早期基因转录水平升高。Ime2的作用将通过确定Ime2是否通过磷酸化直接修饰抑制因子复合体来研究。这将涉及到抑制因子复合体磷酸化位点的改变，并确定其功能是否被改变。突变抑制因子的功能将被测试其关闭和打开其调节基因的能力。此外，Ime2控制早期孢子形成特异性蛋白翻译的能力也将作为控制早期基因的潜在机制进行研究。这将涉及调查是否存在仅当Ime2存在时才翻译的早期基因mrna。通过改变IME1启动子并确定哪些部分对呼吸能力作出反应，将研究呼吸和IME1基因激活之间的联系。这种关系将通过基因筛选进一步研究，以确定在呼吸能力细胞中诱导IME1或在呼吸功能不全细胞中抑制IME1的因素。实现本提案的目标将为理解细胞分化过程中的转录调控提供一个模型，并提供细胞分化过程中转录和翻译调控的分析。

In response to starvation the budding yeast, Saccharomyces cerevisiae, stops growing and dividing and enters into a process referred to as sporulation that leads to the production of haploid spores. The spores are resistant to harsh environmental conditions allowing the cells to survive for long periods of starvation or drought. Upon refeeding the spores can germinate as haploid cells and then engage in a mating process. Thus sporulation is required both for survival in harsh conditions and for sexual reproduction. The sporulation process requires a set of genes distinct from those expressed when the cells are actively growing and dividing. These sporulation-specific genes can be divided into families based upon the timing of their expression. The early genes are necessary for the initiation of the sporulation process. These genes are turned off during growth by the proteins Ume6, Sin3 and Rpd3. Upon starvation the IME1 gene is induced and the Ime1 protein binds to Ume6 to turn on the other early genes. One of these is IME2. When Ime2 protein accumulates it fully turns on the early genes. The IME1 gene is activated in response to starvation but only in cells that have fully functional mitochondria and are capable of respiration. Defective mitochondria or inhibition of respiration block IME1 transcription and the entire sporulation process. The objectives of this proposal are to determine how respiration competence regulates expression of the IME1 gene, and how Ime2 acts to drive elevated levels of early gene transcription required for sporulation. The role of Ime2 will be investigated by determining whether Ime2 directly modifies the repressor complex by phosphorylation. This will involve changing sites of phosphorylation in the repressor complex and determining whether its function is altered. The function of the mutated repressor will be tested for both its ability to turn off and turn on genes that it regulates. Additionally, The ability of Ime2 to control the translation of early sporulation-specific proteins will also be investigated as a potential mechanism that controls early genes. This will involve investigating whether there are any early gene mRNAs that are translated only when Ime2 is present. The link between respiration and activation of the IME1 gene will be investigated by making changes to the IME1 promoter and determining what parts respond to respiration competence. The relationship will be further investigated through genetic screens to identify factors that induce IME1 in respiration competent cells or repress IME1 in respiration incompetent cells. Realization of the objectives of this proposal will provide a model for understanding transcriptional regulation during cellular differentiation and provide an analysis of transcription and translational regulation in cellular differentiation.