site-specific recombination and replicon stability

在这个研究项目中，我们希望研究细菌的遗传物质在细胞分裂之前是如何分离的。为了实现这一点，细胞必须确保在分裂过程结束之前，它的DNA已经完全复制并均匀地划分到每个子细胞中。在这个过程中经常出现的一个问题是在细胞分裂之前2个子染色体的重组，导致染色体二聚体的形成。如果这条双染色体不能有效地分离，子细胞就不会得到相同的细菌基因组拷贝，从而导致细胞死亡。为了克服这个问题，细菌设计了一种方法，通过一种称为位点特异性重组的遗传过程，将双染色体重新转化为两个独立的、相同的副本。这一过程确保重组事件发生在正确的时间（细胞分裂之前）和正确的位置（在细胞间隔，新形成的子细胞之间的连接点）。在大多数细菌中，这一过程是由两种细胞蛋白XerC和XerD完成的，它们在染色体上一个叫做dif的特定位点协同作用。最近，研究表明，某些细菌，如链球菌、乳球菌、幽门螺杆菌和弯曲杆菌，使用单一的Xer蛋白来完成相同的过程。我们感兴趣的是“双”XerCD系统和“单”Xer系统（称为XerS/H）如何在分子水平上工作（蛋白质如何结合DNA，哪些碱基是关键的，以及DNA结合后重组反应如何进行）。这些单重组酶和双重组酶系统似乎进化得很不同，我们最近发现某些细菌同时拥有单重组酶和双重组酶系统。我们感兴趣的是确定这些系统是如何进化的，并且，在同时拥有这两个系统的细菌的情况下，找出哪一个是必不可少的。我们工作的主旨是充分理解这个重要的细胞过程是如何在分子水平上工作的，以及它是如何在时间和空间上受到调节的。

In this research program, we wish to examine how a bacteria's genetic material separates before the cell divides. In order for this to happen, the cell must ensure that its DNA has completely replicated and has been equally partitioned into each daughter cell before the division process terminates. A problem often arises during this process is the recombination of the 2 daughter chromosomes prior to cell division, leading to the formation of a chromosome dimer. If this double chromosome cannot be effectively separated, the daughter cells will not receive identical copies of the bacterial genome, leading to cell death. To overcome this problem, bacteria have devised a way to reconvert double chromosomes into 2 single, independant, identical copies using a genetic process called site-specific recombination. This process ensures tha tthis recombination event occurs at the right time (prior to cell division) and at the right place (at the cell septum, the junction point between the newly formed daughter cells). In the majority of bacteria, this process is performed by 2 cellular proteins, XerC and XerD, which act in concert at a specific chromosomal site called dif. Recently, it was shown that certain bacteria like Streptococci, Lactococci, Helicobacter and Campylobacter use a single Xer protein to perform this identical process. We are interested in how the 'double' XerCD system, and the 'single' Xer system (called XerS/H) work at the molecular level (how the proteins bind DNA, what bases are critical, and how the recombination reaction proceeds after DNA binding). These single and double recombinase systems seem to have evolved divergently, and we have recently found that certain bacteria possess both single and double recombinase systems. We are interested in determining how these systems have evolved, and, in the case of bacteria which possess both systems, finding out which one is essential. The main thrust of our work is to fully understand how this important cellular process works at the molecular level, and how it is regulated both temporally and spatially.