Domesticated conjugation machinery promotes DNA exchange in hyperthermophilic archaea

Conjugation, a major mechanism of horizontal gene transfer promoting the spread of antibiotic resistance among human pathogens, involves establishing a junction between a donor and a recipient cell. The mating pilus plays a prominent role in this process. In bacteria, the conjugation machinery is encoded by plasmids or transposons and typically mediates the transfer of cognate mobile genetic elements. Much less has been known about conjugation in archaea, with no homologs of bacterial mating pilins having been identified in sequence searches. Here, using cryo-electron microscopy, we determined high-resolution structures of three conjugative pili, two from hyperthermophilic archaea, Aeropyrum pernix and Pyrobaculum calidifontis, and one encoded by the Ti plasmid of the bacterium Agrobacterium tumefaciens. We show that the archaeal pili are homologous to bacterial mating pili and display intricate pilin-lipid interaction networks. Unlike in bacteria, the conjugation machinery in hyperthermophilic archaea has been domesticated and mediates the transfer of cellular DNA. We suggest that the constitutive expression of the pili may be due to the chronic damage of DNA at high temperatures that is constantly being repaired using homologous recombination.