REGULATION OF THE ENDO CELL CYCLE IN DROSOPHILA

细胞分裂必须被精确地调控，因为当这种调控失效时，不受控制的分裂和癌症就会随之而来。在多细胞生物的发育过程中，细胞的生长和分裂必须与发育信号相协调。除了从细胞周期中退出以允许分化之外，植物和动物王国中还利用了各种细胞周期变体。其中一种被修饰的细胞周期，即内切细胞周期，在没有有丝分裂的情况下导致DNA复制，并产生多倍体或多烯细胞，这种细胞在几乎所有生物体的某些组织中都存在。本研究以果蝇为模型，探讨了内循环调控的三个关键方面。内切周期和有丝分裂周期最显著的区别是没有有丝分裂。桑葚胚基因是抑制内切循环中有丝分裂所必需的。MORULA蛋白将被鉴定并确定其阻断有丝分裂的机制。两个细胞周期之间的第二个区别是，DNA复制在内切周期中受到不同的调节，因此一些基因组区域复制不足，而另一些区域复制过度，基因扩增。E2F转录因子的两个亚基dDP和dE2F的突变影响多倍体S期的差异复制。这些结果揭示了E2F在S期开始后控制DNA复制的一个以前未被认识到的作用。我们将确定E2F是通过其转录靶标之一影响DNA复制，还是对复制起点有更直接的影响。由于控制E2F活性的调控层次在大多数人类肿瘤中发生突变，因此确定E2F如何影响DNA复制对于了解癌症的原因具有重要意义。第三个水平的调控是内切细胞周期和有丝分裂周期共同的：在DNA复制开始后，在G1-S转变中诱导的转录本必须下调，以便进行下一轮复制。l(2)52Ec基因在DNA复制和G1-S转录物下调中都是必需的，这表明它是细胞识别复制已经开始的先决条件。l(2)51Ec蛋白是起源复合体的一个组成部分，并且是先前诱导基因下调所必需的假设将被验证。

Cell division must be regulated precisely, because when this regulation fails uncontrolled division and cancer ensue. During the development of multicellular organisms cell growth and division have to be coordinated with developmental signals. In addition to exit from the cell cycle to permit differentiation, a variety of cell cycle variants are utilized throughout the plant and animal kingdoms. One of these modified cell cycles, the endo cell cycle, results in DNA replication in the absence of mitosis and produces the polyploid or polytene cells that are found in at least some tissues in almost all organisms. This proposal addresses the three key aspects of how the endo cycle is regulated, using Drosophila as a model. The most significant distinction between the endo cycle and the mitotic cycle is the absence of mitosis. The morula gene is required to inhibit mitosis in the endo cycle. The MORULA protein will be identified and the mechanism by which it blocks mitosis determined. A second difference between the two cell cycles is that DNA replication is differentially regulated in the endo cycle such that some genomic regions are underreplicated while others are overreplicated and the genes amplified. Mutations in the two subunits of the E2F transcription factor, dDP and dE2F, affect differential replication during polyploid S phase. These results reveal a previously unrecognized role for E2F in controlling DNA replication once S phase has initiated. It will be determined whether E2F affects DNA replication via one of its transcriptional targets or whether it has a more direct effect on replication origins. Because the regulatory hierarchy that controls E2F activity is mutated in most human tumors, it is significant for understanding the causes of caner to determine how E2F influences DNA replication. The third level of regulation is common to both the endo cell cycle and the mitotic cycle: the transcripts that are induced at the G1-S transition must be downregulated after DNA replication has initiated in order for a subsequent round of replication to occur. The l(2)52Ec gene is unusual in being necessary both for DNA replication and downregulation of G1-S transcripts, suggesting it is a prerequisite for the cell to recognize that replication has initiated. The hypothesis that the l(2)51Ec protein is a component of the origin complex and necessary for downregulation of previously induced genes will be tested.