CDI Systems Are Stably Maintained by a Cell-Contact Mediated Surveillance Mechanism.
CDI Systems Are Stably Maintained by a Cell-Contact Mediated Surveillance Mechanism.
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DOI：
10.1371/journal.pgen.1006145
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发表时间：
2016-06
期刊：
影响因子：
4.5
通讯作者：
Low DA
中科院分区：
文献类型：
--
作者：
Ruhe ZC;Nguyen JY;Chen AJ;Leung NY;Hayes CS;Low DA
接触依赖性生长抑制（CDI）系统广泛存在于革兰氏阴性菌中，它们在细胞间竞争和生物膜形成中发挥重要作用。CDI+细菌使用细胞表面CdiA蛋白结合邻近细菌并递送C末端毒素结构域。CDI+细胞还表达特异性中和来自相邻同胞的毒素的CDII免疫蛋白。基因组分析表明，cdi位点通常发现质粒和基因组岛，这表明这些5型分泌系统通过水平基因转移传播。在这里，我们研究是否CDI毒素和免疫活动的服务，以稳定移动的遗传因素，使用最小的F质粒，未能在细胞分裂过程中正确分区。该F质粒在50代细胞内从大肠杆菌群体中丢失，但当该质粒携带来自大肠杆菌的cdi基因簇时，在100代后仍保留在约60%的细胞中。coli EC93。相比之下，通常在F上发现的ccdAB“质粒成瘾”模块仅发挥适度的稳定作用。依赖于CDI的质粒稳定化需要CDI A的BamA受体，这表明无质粒的子细胞被保留CDI+质粒的同胞抑制。为了支持该模型，CDI+ F质粒从在单独的质粒上携带额外的cdiI免疫基因的细胞中迅速丢失。这些结果表明，质粒稳定化通过质粒丢失消除群体中产生的非免疫细胞而发生。因此，遗传稳定反映了对CDI免疫的强选择。在长期传代超过300代后，CDI+质粒获得增加拷贝数的突变，并导致群体中100%携带。总之，这些结果表明，CDI通过毒素介导的监视机制稳定遗传元件，在该机制中，失去CDI系统的细胞被其兄弟姐妹检测和消除。许多细菌表达接触依赖性生长抑制（CDI）系统，其用于将相同物种的细菌结合在一起并递送阻止细菌生长的毒素。同胞细胞受到CDI编码的免疫蛋白的保护，但不相关的邻居由于缺乏免疫力而受到抑制。因此，CDI提供了一种将相邻小区识别为“自身”或“非自身"的机制。CDI基因通常存在于可以在不同细菌之间移动的基因组DNA区域上。在这里，我们发现CDI基因促进了称为质粒的小复制DNA元件的稳定性，使它们能够在种群中维持许多代。如果一个细胞失去了CDI质粒DNA，那么它也失去了对毒素的免疫力，并被邻近的CDI+细胞消除。我们称之为监视机制，因为CDI+细胞不断释放毒素来测试它们的邻居是否是真正的兄弟姐妹。缺乏CDI基因的细胞被认为是外来的，并从群体中消除。这项工作表明，CDI系统施加了强大的选择压力，并发挥作用，以稳定与它们相关的DNA元件。
Contact-dependent growth inhibition (CDI) systems are widespread amongst Gram-negative bacteria where they play important roles in inter-cellular competition and biofilm formation. CDI+ bacteria use cell-surface CdiA proteins to bind neighboring bacteria and deliver C-terminal toxin domains. CDI+ cells also express CdiI immunity proteins that specifically neutralize toxins delivered from adjacent siblings. Genomic analyses indicate that cdi loci are commonly found on plasmids and genomic islands, suggesting that these Type 5 secretion systems are spread through horizontal gene transfer. Here, we examine whether CDI toxin and immunity activities serve to stabilize mobile genetic elements using a minimal F plasmid that fails to partition properly during cell division. This F plasmid is lost from Escherichia coli populations within 50 cell generations, but is maintained in ~60% of the cells after 100 generations when the plasmid carries the cdi gene cluster from E. coli strain EC93. By contrast, the ccdAB "plasmid addiction" module normally found on F exerts only a modest stabilizing effect. cdi-dependent plasmid stabilization requires the BamA receptor for CdiA, suggesting that plasmid-free daughter cells are inhibited by siblings that retain the CDI+ plasmid. In support of this model, the CDI+ F plasmid is lost rapidly from cells that carry an additional cdiI immunity gene on a separate plasmid. These results indicate that plasmid stabilization occurs through elimination of non-immune cells arising in the population via plasmid loss. Thus, genetic stabilization reflects a strong selection for immunity to CDI. After long-term passage for more than 300 generations, CDI+ plasmids acquire mutations that increase copy number and result in 100% carriage in the population. Together, these results show that CDI stabilizes genetic elements through a toxin-mediated surveillance mechanism in which cells that lose the CDI system are detected and eliminated by their siblings. Many bacteria express contact-dependent growth inhibition (CDI) systems, which are used to bind bacteria of the same species together and deliver toxins that block bacterial growth. Sibling cells are protected by a CDI-encoded immunity protein, but unrelated neighbors are inhibited because they lack immunity. Thus, CDI provides a mechanism to identify neighboring cells as "self" or "nonself-". CDI genes are typically found on genomic DNA regions that can move between different bacteria. Here, we find that CDI genes promote the stability of small replicating DNA elements called plasmids, allowing them to be maintained in a population over many generations. If a cell loses the CDI plasmid DNA, then it also loses immunity to toxin and is eliminated by neighboring CDI+ cells. We call this a surveillance mechanism because CDI+ cells continually deliver toxins to test whether their neighbors are true siblings. Cells lacking the CDI genes are recognized as foreign and are eliminated from the population. This work shows that CDI systems exert a powerful selective pressure and act to stabilize DNA elements with which they are associated.