Relaxases and Plasmid Transfer in Gram-Negative Bacteria.

All plasmids that spread by conjugative transfer encode a relaxase. That includes plasmids that encode the type IV secretion machinery necessary to mediate cell to cell transfer, as well as mobilizable plasmids that exploit the existence of other plasmids' type IV secretion machinery to enable their own lateral spread. Relaxases perform key functions in plasmid transfer by first binding to their cognate plasmid as part of a multiprotein complex called the relaxosome, which is then specifically recognized by a receptor protein at the opening of the secretion channel. Relaxases catalyze a site- and DNA-strand-specific cleavage reaction on the plasmid then pilot the single strand of plasmid DNA through the membrane-spanning type IV secretion channel as a nucleoprotein complex. In the recipient cell, relaxases help terminate the transfer process efficiently and stabilize the incoming plasmid DNA. Here, we review the well-studied MOB family of relaxases to describe the biochemistry of these versatile enzymes and integrate current knowledge into a mechanistic model of plasmid transfer in Gram-negative bacteria.