Barton Ian S, Eagan Justin L, Nieves-Otero Priscila A, Reynolds Ian P, Platt Thomas G, Fuqua Clay
Department of Biology, Indiana University, Bloomington, IN, United States.
Division of Biology, Kansas State University, Manhattan, KS, United States.
Front Microbiol. 2021 Jan 18;11:605896. doi: 10.3389/fmicb.2020.605896. eCollection 2020.
Members of the , often carry multiple secondary replicons in addition to the primary chromosome with compatible -based replication systems. Unlike secondary chromosomes and chromids, -based megaplasmids and plasmids can undergo copy number fluctuations and are capable of conjugative transfer in response to environmental signals. Several lineages harbor three secondary -based replicons, including a secondary chromosome (often linear), the Ti (tumor-inducing) plasmid and the At megaplasmid. The Ti plasmid is required for virulence and encodes a conjugative transfer () system that is strictly regulated by a subset of plant-tumor released opines and a well-described acyl-homoserine lactone (AHL)-based quorum-sensing mechanism. The At plasmids are generally not required for virulence, but carry genes that enhance rhizosphere survival, and these plasmids are often conjugatively proficient. We report that the At megaplasmid of the octopine-type strain 15955 encodes a quorum-controlled conjugation system that directly interacts with the paralogous quorum sensing system on the co-resident Ti plasmid. Both the pAt15955 and pTi15955 plasmids carry homologs of a TraI-type AHL synthase, a TraR-type AHL-responsive transcription activator, and a TraM-type anti-activator. The genes from both pTi15955 and pAt15955 can direct production of the inducing AHL (3-octanoyl-L-homoserine lactone) and together contribute to the overall AHL pool. The TraR protein encoded on each plasmid activates AHL-responsive transcription of target gene promoters. The pAt15955 TraR can cross-activate genes on the Ti plasmid as strongly as its cognate genes, whereas the pTi15955 TraR is preferentially biased toward its own genes. Putative box elements are located upstream of target promoters, and comparing between plasmids, they are in similar locations and share an inverted repeat structure, but have distinct consensus sequences. The two AHL quorum sensing systems have a combinatorial effect on conjugative transfer of both plasmids. Overall, the interactions described here have implications for the horizontal transfer and evolutionary stability of both plasmids and, in a broad sense, are consistent with other systems that often have multiple quorum-sensing controlled secondary replicons.
根癌土壤杆菌属的成员，除了具有基于共生质粒的复制系统的主染色体外，通常还携带多个次生复制子。与次生染色体和染色质不同，基于共生质粒的大质粒和质粒能够经历拷贝数波动，并且能够响应环境信号进行接合转移。几个根癌土壤杆菌谱系含有三个基于共生质粒的复制子，包括一条次生染色体（通常是线性的）、Ti（致瘤）质粒和At大质粒。Ti质粒是致病所必需的，它编码一个接合转移（Tra）系统，该系统受到植物肿瘤释放的一类冠瘿碱以及一种已被充分描述的基于酰基高丝氨酸内酯（AHL）的群体感应机制的严格调控。At质粒通常不是致病所必需的，但携带增强根际生存能力的基因，并且这些质粒通常具有接合能力。我们报道，章鱼碱型菌株15955的At大质粒编码一个群体感应控制的接合系统，该系统与共居的Ti质粒上的同源群体感应系统直接相互作用。pAt15955和pTi15955质粒都携带TraI型AHL合酶、TraR型AHL响应转录激活因子和TraM型抗激活因子的同源物。来自pTi15955和pAt15955的这些基因都能指导诱导性AHL（3-辛酰-L-高丝氨酸内酯）的产生，并共同对整体AHL库做出贡献。每个质粒上编码的TraR蛋白激活靶标tra基因启动子的AHL响应转录。pAt15955 TraR能够像其同源tra基因一样强烈地反式激活Ti质粒上的tra基因，而pTi15955 TraR则优先偏向于其自身的tra基因。假定的tra盒元件位于靶标启动子的上游，比较不同质粒时，它们处于相似位置并共享一个反向重复结构，但具有不同的共有序列。这两个AHL群体感应系统对两种质粒的接合转移具有组合效应。总体而言，这里描述的相互作用对两种质粒的水平转移和进化稳定性具有影响，并且从广义上讲，与其他通常具有多个群体感应控制的次生复制子的根癌土壤杆菌系统是一致的。
