Department of Microbiology and Immunology, University of North Carolina-Chapel Hill, NC, United States of America.
PLoS Genet. 2019 Jan 7;15(1):e1007883. doi: 10.1371/journal.pgen.1007883. eCollection 2019 Jan.
Intercellular communication and self-recognition are critical for coordinating cooperative and competitive behaviors during sociomicrobiological community development. Contact-dependent growth inhibition (CDI) proteins are polymorphic toxin delivery systems that inhibit the growth of non-self neighboring bacteria that lack the appropriate immunity protein. In Burkholderia thailandensis, CDI system proteins (encoded by bcpAIOB genes) also induce cooperative behaviors among sibling (self) cells, a phenomenon called contact-dependent signaling (CDS). Here we describe a mobile genetic element (MGE) that carries the bcpAIOB genes in B. thailandensis E264. It is a ~210 kb composite transposon with insertion sequence (IS) elements at each end. Although the ISs are most similar to IS2 of Escherichia coli, the transposase-dependent intermediate molecule displays characteristics more similar to those of the IS26 translocatable unit (TU). A reaction requiring only the "left" IS-encoded transposase results in formation of an extrachromosomal circular dsDNA intermediate ("the megacircle") composed of the left IS and the sequences intervening between the ISs. Insertion of the megacircle into the chromosome occurs next to a pre-existing copy of an IS2-like element, recreating a functional composite transposon. We found that BcpA activity is required for megacircle formation, and in turn, megacircle formation is required for CDS phenotypes. Our data support a model in which the bcpAIOB genes function as both helping and harming greenbeard genes, simultaneously enhancing the fitness of self bacteria that possess the same allele plus tightly linked genes that mediate cooperative behaviors, and killing non-self bacteria that do not possess the same bcpAIOB allele. Mobility of the megacircle between cells could allow bacteria invading a community to be converted to self, and would facilitate propagation of the bcpAIOB genes in the event that the invading strain is capable of overtaking the resident community.
细胞间通讯和自我识别对于协调社会微生物群落发展过程中的合作和竞争行为至关重要。接触依赖性生长抑制（CDI）蛋白是多态性毒素输送系统，可抑制缺乏适当免疫蛋白的非自身邻近细菌的生长。在伯克霍尔德氏菌中，CDI 系统蛋白（由 bcpAIOB 基因编码）还诱导同种（自我）细胞之间的合作行为，这种现象称为接触依赖性信号（CDS）。在这里，我们描述了一个在 B. thailandensis E264 中携带 bcpAIOB 基因的移动遗传元件（MGE）。它是一个约 210kb 的复合转座子，两端都有插入序列（IS）元件。虽然 IS 最类似于大肠杆菌的 IS2，但转座酶依赖性中间分子的特征更类似于 IS26 可移动单元（TU）。仅需要“左”IS 编码转座酶的反应导致形成一个额外的染色体圆形双链 DNA 中间物（“大环”），由左 IS 和 IS 之间的序列组成。大环插入染色体发生在一个预先存在的 IS2 样元件的旁边，重新创建一个功能性复合转座子。我们发现 BcpA 活性是大环形成所必需的，反过来，大环形成是 CDS 表型所必需的。我们的数据支持这样一种模型，即 bcpAIOB 基因同时作为帮助和伤害“绿胡子”基因发挥作用，同时增强了具有相同等位基因和紧密连接基因的自我细菌的适应性，这些基因介导合作行为，杀死不具有相同 bcpAIOB 等位基因的非自我细菌。大环在细胞之间的移动性可以使入侵群落的细菌转化为自我，并且在入侵菌株能够超越常驻群落的情况下，有利于 bcpAIOB 基因的传播。
