Structural biology of the Gram-negative bacterial conjugation systems

•Conjugation, a process by which DNA is transported from one bacterium to another, is mediated by type IV secretion systems (T4SSs).•T4SSs span the cell envelope and comprise 12 proteins, including 3 ATPases assembling into a ∼4 MDa complex.•The plasmid DNA is processed in the cytoplasm by the relaxosome, a complex containing the relaxase protein.•The DNA–relaxase complex is recruited to the T4SS, through which it is transported into a recipient cell.

Conjugation, the process by which plasmid DNA is transferred from one bacterium to another, is mediated by type IV secretion systems (T4SSs). T4SSs are versatile systems that can transport not only DNA, but also toxins and effector proteins. Conjugative T4SSs comprise 12 proteins named VirB1–11 and VirD4 that assemble into a large membrane-spanning exporting machine. Before being transported, the DNA substrate is first processed on the cytoplasmic side by a complex called the relaxosome. The substrate is then targeted to the T4SS for export into a recipient cell. In this review, we describe the recent progress made in the structural biology of both the relaxosome and the T4SS.