MOLECULAR MECHANISMS OF DIFFERENTIATION

生物学中最具挑战性的问题是确定 不同的细胞类型是由相同的基因组产生的。 作为 模型系统，以了解涉及的机制， 在真核细胞的变化，我们已经利用了一个系统， 裂殖酵母中交配型基因座的遗传转换， 裂殖酵母 P（正）和M（负）交配 类型S. pombe交换到其他细胞类型， 转座-取代反应，其中任一个的拷贝 两个未表达的供体基因座，mat 2-P和mat 3-M， 到表达的位点，mat 1。 一个引人注目的观察是， 在一对姐妹细胞中，92%的细胞分裂 两个单元中的仅一个单元生成单个切换单元， 下一代 我们最近的研究表明， 细胞谱系中的转换是由于“遗传性”染色体 修饰，使得一些染色体被增强， 而其他人则没有。 此外，我们还表明， 子细胞之间的发育不对称是一种 遗传不等价的亲本DNA链的结果。 我们的目标是确定“半遗传”的分子本质 DNA“修饰”和随后的事件所需的 切换 我们将直接测试限制-修改 我们提出的用于启动MAT 1切换事件的模型， 其由位点特异性双链DNA断裂组成。 更具体地说，我们将描述开关的功能 (swi)基因，我们认为这是必要的， 修饰以及DNA切割事件，使用两种遗传学方法， 和分子技术。 断裂的性质和 修饰和DNA切割所需的序列将被 定义了 了解分子的本质 导致S. 粟酒裂殖酵母可能具有一般的生物学相关性， 所有的实核生物都应该执行这样的程序。

A most challenging problem in biology is to detemine how different cell types are generated from the same genome. As a model system to understand the mechanisms involved in directed changes of cell in a eucaryote, we have exploited a system of heribale swithches of the mating type locus in the fission yeast, Schizosaccharomyces pombe. The P (plus) and M (minus) mating types of S. pombe interchange to the other cell type by a transposition-substitution reaction in which a copy of either one of two unexpressed donor loci, mat2-P and mat3-M, is transmitted to the expressed locus, mat 1. One striking observation is that within a pair of sister cells, in as many as 92% of cell divisions only one of the two cells generates a single switched cell in the next generation. We have recently shown that this pattern of switching in a cell lineage is due to an "epidgenetic" chromosomal modification such that some chromosomes are potentiated to switch whereas others are not. Furthermore, we have shown that the developmental asymmetry among daughter cells is a consequence of inheriting non-equivalent parental DNA chains. Our goal is to define the molecular nature of the "semi-heritable" DNA "modification" and the subsequent events required for switching. We will directly test the restriction-modification model we have proposed for initiating the mat 1 switching event, which is comprised of a site-specific double stranded DNA break. More specifically, we will characterize the function of the switch (swi) genes, which we have suggested to be required for the modification as well as DNA cleavage events, using both genetic and molecular techniques. The nature of the break and the sequences required for moditication and DNA cleavage will be defined. Understanding the molecular nature of the developmental program that leads to cellular differentiation in S. pombe can be expected to have general biological relevance since all ercaryotes are also expected to execute such programs.