Chimeric systems composed of swapped Tra subunits between distantly-related F plasmids reveal striking plasticity among type IV secretion machines.

Mobile genetic elements (MGEs) comprise a diverse group of extrachromosomal plasmids or integrated DNA fragments that are widely distributed among many bacterial species. MGEs typically encode conjugation systems dedicated to their transmission to other bacteria, and also code for resistance to antibiotics or virulence or other fitness traits. The conjugation systems, along with an equally medically important group of translocators devoted to the interkingdom delivery of protein effectors by pathogenic species, comprise the superfamily of type IV secretion systems (T4SSs). Recent studies have defined many mechanistic and structural features of the T4SSs, yet there remains little understanding of how T4SSs recruit specific DNA or protein substrates, elaborate functional channels, and in some cases build attachment organelles termed conjugative pili. We explored the mechanics of T4SS machine function by systematically exchanging individual components between two distinct conjugation systems functioning in enterobacterial species. Through construction of chimeric machines, and further mutational analyses, we identified subunits or protein domains of conjugation machines specifying recruitment of distinct DNA substrates or selectively contributing to assembly of translocation channels or conjugative pili. Such features of T4SSs are prime targets for development of inhibitory strategies aimed at blocking T4SS functions for therapeutic intervention.