Laboratory of Biochemistry and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, United States of America.
PLoS Genet. 2018 May 24;14(5):e1007426. doi: 10.1371/journal.pgen.1007426. eCollection 2018 May.
Initiation of chromosome replication in bacteria is precisely timed in the cell cycle. Bacteria that harbor multiple chromosomes face the additional challenge of orchestrating replication initiation of different chromosomes. In Vibrio cholerae, the smaller of its two chromosomes, Chr2, initiates replication after Chr1 such that both chromosomes terminate replication synchronously. The delay is due to the dependence of Chr2 initiation on the replication of a site, crtS, on Chr1. The mechanism by which replication of crtS allows Chr2 replication remains unclear. Here, we show that blocking Chr1 replication indeed blocks Chr2 replication, but providing an extra crtS copy in replication-blocked Chr1 permitted Chr2 replication. This demonstrates that unreplicated crtS copies have significant activity, and suggests that a role of replication is to double the copy number of the site that sufficiently increases its activity for licensing Chr2 replication. We further show that crtS activity promotes the Chr2-specific initiator function and that this activity is required in every cell cycle, as would be expected of a cell-cycle regulator. This study reveals how increase of gene dosage through replication can be utilized in a critical regulatory switch.
细菌中染色体复制的启动在细胞周期中是精确定时的。拥有多个染色体的细菌面临着协调不同染色体复制起始的额外挑战。在霍乱弧菌中，其两个染色体中的较小染色体 Chr2 在 Chr1 之后开始复制，使得两条染色体同步终止复制。这种延迟是由于 Chr2 起始复制依赖于 Chr1 上的一个位点 crtS 的复制。复制 crtS 如何允许 Chr2 复制的机制仍不清楚。在这里，我们表明阻断 Chr1 复制确实会阻断 Chr2 复制，但在复制受阻的 Chr1 中提供额外的 crtS 拷贝允许 Chr2 复制。这表明未复制的 crtS 拷贝具有显著的活性，并表明复制的作用是将该位点的拷贝数翻倍，从而显著增加其活性，以许可 Chr2 复制。我们进一步表明 crtS 活性促进 Chr2 特异性起始功能，并且这种活性在每个细胞周期中都是必需的，这是细胞周期调节剂的预期作用。这项研究揭示了通过复制增加基因剂量如何在关键调控开关中得到利用。
