Studies of diversification of plasmid replication regions by genetic recombination

生物是由各种基因高度组织起来的信息系统。在这个项目中，我们研究了一些细菌质粒的复制系统在进化过程中是如何分化的。细菌质粒的基本复制子由复制结构系统和复制控制系统两套遗传系统组成。核苷酸序列比较表明，质粒P307 （IncFI）和pMU2200 （IncZ）是由R100 （IncFII）和ColIb-P9 （incalpha）的祖先相互重组产生的，反之亦然。每对质粒P307/pMU2200和R100/ColIb-P9在结构上互不相关。基于这些信息，我们在体外构建并分析了ColIb-P9和R100的p307样嵌合复制子。当ColIb-P9的复制结构区与R100的整个复制控制区结合时，得到的复制子与R100一样稳定地复制。这些结果表明，质粒的基本复制子是通过交换它们的复制控制系统而分离的。因此，我们提出，在某些情况下，质粒的复制控制系统独立于它们的结构系统进化，尽管这两个系统一起工作以维持复制功能。我们还研究了相互重组是由参与控制复制启动蛋白编码基因表达的RNA的独特二级结构指定的。

Living things are information systems that are highly organized by a variety of genes. In this project, we have studied how replication systems of some bacterial plasmids diverged during the course of evolution. The basic replicons of bacterial plasmids consist of two sets of genetic systems, the replication-structural system and the replication control system. Comparison of nucleotide sequences suggested that plasmids P307 (IncFI) and pMU2200 (IncZ) were generated by reciprocal recombination between ancestors of R100 (IncFII) and ColIb-P9 (IncIalpha) , or vice versa. The plasmids of each pair, P307/pMU2200 and R100/ColIb-P9, are structurally unrelated to each other. Based on this information, we constructed in vitro and analyzed P307-like chimeric replicons from ColIb-P9 and R100. When the replication-structural region of ColIb-P9 was combined with the whole replication control region of R100, the resultant replicon replicated stably as R100 did. These results revealed that the basic replicons of the plasmids diverged by exchanging their replication control systems. Thus, we propose that the replication control systems of plasmids, in some cases, evolved independently of their structural systems, although these two systems work together for maintaining the replication functions. We also studied that the reciprocal recombination was specified by the unique secondary structures of RNA involved in the control of expression of the genes encoding the replication initiator proteins.