Ayub Ow Yong, L.; Yeow, J.; Tiruvayipati, S.; Chen, S.; Cai, C. G. X.; Chen, S. L.; Chng, S.-S.

Many bacterial defense (immune) systems prevent the entry of foreign DNA by directly recognizing and targeting nucleic acids, effectively blocking all mechanisms of horizontal gene transfer. However, systems defending specifically against conjugation, a major route for gene dissemination, have heretofore not been reported. We have discovered a novel defense factor, which we name AbjA (Abortive conjugation protein A), that specifically limits successful plasmid conjugation into a recipient bacterium. AbjA interacts directly with and targets the ATPase component TrbE of the Type IV secretion system (T4SS) to induce cell death; this contrasts with most other defense systems that act at the nucleic acid level. AbjA therefore represents the first member of a new class of bacterial defense factors that trigger what we term "abortive conjugation". Previously, recipient bacteria were viewed largely as defenseless against the mechanism of conjugation; our discovery and characterization of AbjA demonstrates that recipient bacteria can block conjugation to limit the transfer (and thus spread) of plasmids. Discovery of this class of defense systems thus has implications for bacterial defense, plasmid evolution, and possible strategic alternatives to rationally target plasmid spread, particularly with respect to virulence and antibiotic resistance.