Vibrio cholerae, the pathogenic bacterium that causes cholera, has two chromosomes (Chr1, Chr2) that replicate in a well-orchestrated sequence. Chr2 initiation is triggered only after the replication of the crtS site on Chr1. The initiator of Chr2 replication, RctB, displays activities corresponding with its different binding sites: initiator at the iteron sites, repressor at the 39m sites, and trigger at the crtS site. The mechanism by which RctB relays the signal to initiate Chr2 replication from crtS is not well-understood. In this study, we provide new insights into how Chr2 replication initiation is regulated by crtS via RctB. We show that crtS (on Chr1) acts as an anti-inhibitory site by preventing 39m sites (on Chr2) from repressing initiation. The competition between these two sites for RctB binding is explained by the fact that RctB interacts with crtS and 39m via the same DNA-binding surface. We further show that the extreme C-terminal tail of RctB, essential for RctB self-interaction, is crucial for the control exerted by crtS. This subregion of RctB is conserved in all Vibrio, but absent in other Rep-like initiators. Hence, the coordinated replication of both chromosomes likely results from the acquisition of this unique domain by RctB.

中文翻译：

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霍乱弧菌两条染色体的协调复制需要 RctB 启动子获得一个独特的结构域

霍乱弧菌是引起霍乱的病原菌，它有两条染色体（Chr1、Chr2），它们以精心编排的顺序复制。只有在 Chr1 上的 crtS 位点复制后才会触发 Chr2 启动。Chr2 复制的启动子 RctB 显示与其不同结合位点相对应的活动：启动子位于 iteron 位点，阻遏物位于 39m 位点，触发位点位于 crtS 位点。RctB 传递信号以从 crtS 启动 Chr2 复制的机制尚不清楚。在这项研究中，我们提供了关于 CrtS 如何通过 RctB 调节 Chr2 复制起始的新见解。我们表明 crtS（在 Chr1 上）通过阻止 39m 位点（在 Chr2 上）抑制起始来充当抗抑制位点。这两个位点之间对 RctB 结合的竞争可以通过以下事实来解释：RctB 通过相同的 DNA 结合表面与 crtS 和 39m 相互作用。我们进一步表明，RctB 的极端 C 末端尾部是 RctB 自我交互所必需的，对于 crtS 施加的控制至关重要。RctB 的这个子区域在所有弧菌中都是保守的，但在其他类似 Rep 的引发剂中不存在。因此，两条染色体的协调复制可能是由于 RctB 获得了这个独特的结构域。