TraC of the conjugative plasmid pKM101 is secreted as a monomer in outer membrane vesicles and stimulates conjugation

Abstract

Gram-negative bacteria use membrane-bound type IV secretion systems to assemble pili on the cell surface, followed by cell-cell contact with recipient cells and transfer of plasmid DNA. The process at the cell-cell contact stage of conjugative DNA transfer is not well understood. We here present a biochemical and genetic characterization of the TraC protein that is a minor component of the pili determined by the IncN plasmid pKM101 from Escherichia coli. The cellular and secreted forms of TraC are monomers, TraC preferentially localizes at the cell poles and it is also detected in extracellular membrane vesicles. Purified TraC does not impact the infection with bacteriophages, but we detect binding of TraC to recipient cells and partial complementation of a traC deletion strain by the addition of purified TraC. These results suggest that the protein contributes to conjugation at the cell-cell contact stage.