Structure of TrwB, a gatekeeper in bacterial conjugation.

Bacterial conjugation implies a trans-membrane passage of DNA, mediated by proteins encoded in conjugative plasmids. This results in a spread of genetic information, including antibiotic resistance acquisition by pathogens. Special cases of conjugation are trans-kingdom gene transfer from bacteria to plants or fungi, and even bacterial sporulation and cell division. One of the main actors in this process is an integral inner membrane DNA-binding protein, called TrwB in the E. coli R388 conjugative system. It is responsible for coupling the single-strand DNA to be transferred from the donor to the acceptor cell in its complex with other proteins, with a type IV secretion system making up the mating apparatus. The TrwB protomer consists of two domains: a nucleotide-binding domain of alpha/beta topology, similar to RecA and DNA ring helicases, and an all-alpha domain. The quaternary structure reveals an almost spherical homohexamer, strikingly similar to F(1)-ATPase. A central 20 A wide channel traverses the hexamer, thus connecting cytoplasm with periplasm.