DNA defense systems become a fitness liability during plasmid conjugation

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.