研究发现DNA防御系统在质粒接合中成为适合度负担
DNA defense systems become a fitness liability during plasmid conjugation
研究发现，接合质粒转移会通过致死性接合（lethal zygosis）选择性降低非许可受体的适合度：反复的IV型分泌系统（T4SS）接触会损伤未能建立质粒的受体细胞，从而为质粒供体清出生态位。在Escherichia coli中，用CRISPR–Cas或限制修饰系统阻断质粒RK2的建立会造成明显的适合度缺陷，原因是受体无法表达质粒编码的排斥基因，持续暴露于T4SS接触之下。作者据此开发了共递送排斥基因的转座子载体，将细菌基因编辑效率提高数个数量级。研究提出水平基因转移中的“强制性同化”机制，并指出细菌对外源DNA的免疫优势与更易遭受致死性接合之间存在权衡。
来源：Nucleic Acids Research · doi.org
研究揭示细菌免疫防御在接合质粒转移中反成适合度负担，并据此改造转座子载体提升编辑效率。
Horizontal gene transfer via conjugative plasmids is a major driver of bacterial evolution. While antibiotic exposure selects for the resistance genes carried by some plasmids, much uncertainty remains about how plasmids persist and spread in microbial communities in the absence of such external selection. Here, we show that conjugative plasmid transfer selectively reduces the fitness of non-permissive recipients through the process of lethal zygosis: a phenomenon in which repeated Type IV Secretion System (T4SS) engagement damages recipients that fail to establish the plasmid. We found that this contact-dependent antagonism effectively clears a niche for plasmid donors by turning recipients’ DNA defenses into a liability. Blocking establishment of the plasmid RK2 with CRISPR–Cas or restriction-modification caused strong fitness defects in Escherichia coli recipients by preventing expression of plasmid-encoded exclusion genes and leaving cells exposed to repeated T4SS engagement. We leveraged this insight to resolve an obstacle in bacterial editing by developing transposon vectors that co-deliver exclusion genes, increasing the efficiency of gene editing by orders of magnitude. Our results reveal the role of “coercive assimilation” in horizontal gene transfer, exposing a tradeoff where the advantage of bacterial immunity to foreign DNA is counterbalanced by increased susceptibility to lethal zygosis.