Programa de Genómica Evolutiva, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apartado Postal 565-A, Cuernavaca, Morelos, Mexico.
Plasmid. 2013 Jan;69(1):49-57. doi: 10.1016/j.plasmid.2012.08.003. Epub 2012 Sep 4.
The repABC replication/partitioning systems are commonly found in alpha-proteobacteria plasmids and in secondary chromosomes. All of the elements required for their replication and stable maintenance are encoded within a single transcription unit: the repABC operon. The repC gene encodes an initiator protein, while RepA, RepB and centromere-like sequence (parS) direct plasmid segregation. Strains containing two or more repABC plasmids are a common feature in some alpha proteobacteria groups, indicating that the repABC plasmid family embraces several incompatibility groups. Genes encoded within repABC operons are highly dynamic: each one possess its own distinctive phylogeny and homologous recombination events are common within these operons. Additionally, alpha-proteobacterial genomes contain repAB genes not associated with the ctRNA or with repC as well as plasmids whose replication depends on a ctRNA-repC module without the participation of repAB genes. Some alphaproteobacteria have repC genes clustered with other genes that are not involved in replication/partitioning functions. These atypical associations of genes could have an important role in the origin and diversification of new plasmids. Here we evaluated the functionality and possible evolutionary consequences of one of these atypical gene associations: the repAC genes present in the Rhizobium leguminosarum plasmid pRL7. The repAC genes are organized in an operon and they are capable of sustaining replication but in an unstable manner. RepC was essential for replication, and the origin of replication resides within its coding region. In contrast, RepA plays a minor role in the negative regulation of its own transcription.
repABC 复制/分配系统通常存在于α-变形菌质粒和次级染色体中。它们复制和稳定维持所需的所有元件都编码在单个转录单元中：repABC 操纵子。repC 基因编码起始蛋白，而 RepA、RepB 和类似着丝粒序列（parS）则指导质粒分离。含有两个或更多 repABC 质粒的菌株是一些α变形菌群的常见特征，这表明 repABC 质粒家族包含几个不相容群。repABC 操纵子内编码的基因具有高度的动态性：每个基因都有其独特的系统发育，并且这些操纵子内同源重组事件很常见。此外，α-变形菌基因组包含与 ctRNA 或 repC 无关的 repAB 基因，以及依赖于 ctRNA-repC 模块而不参与 repAB 基因的复制的质粒。一些α变形菌具有与复制/分配功能无关的 repC 基因与其他基因聚类。这些基因的非典型关联可能在新质粒的起源和多样化中发挥重要作用。在这里，我们评估了其中一种非典型基因关联的功能和可能的进化后果：存在于根瘤菌属 pRL7 质粒中的 repAC 基因。repAC 基因在操纵子中组织，能够维持不稳定的复制。RepC 对复制是必需的，复制起点位于其编码区。相比之下，RepA 在其自身转录的负调控中仅起次要作用。
