The bacterial conjugation protein TrwB resembles ring helicases and F-1-ATPase

The transfer of DNA across membranes and between cells is a central biologi cal process; however, its molecular mechanism remains unknown. In prokaryot es, trans-membrane passage by bacterial conjugation, is the main route for horizontal gene transfer. It is the means for rapid acquisition of new gene tic information, including antibiotic resistance by pathogens. Transkingdom gene transfer from bacteria to plants(1) or fungi(2) and even bacterial sp orulation(3) are special cases of conjugation. An integral membrane DNA-bin ding protein, called TrwB in the Escherichia coli R388 conjugative system, is essential for the conjugation process. This large multimeric protein is responsible for recruiting the relaxosome DNA-protein complex, and particip ates in the transfer of a single DNA strand during cell mating. Here we rep ort the three-dimensional structure of a soluble variant of TrwB. The molec ule consists of two domains: a nucleotide-binding domain of alpha/beta topo logy, reminiscent of RecA and DNA ring helicases, and an all-alpha domain. Six equivalent protein monomers associate to form an almost spherical quate rnary structure that is strikingly similar to F-1-ATPase. A central channel , 20 Angstrom in width, traverses the hexamer.