The F plasmid transfer activator TraJ is a dimeric helix-turn-helix DNA-binding protein

TraJ is an activator of the transfer (tra) operon in the F plasmid that counteracts H-NS silencing at the main transfer promoter (P-Y). TraJ contains 226 aa (26 670 kDa), not 229 aa as reported previously, and forms homodimers. TraJ binds DNA containing P-Y in vivo as demonstrated using a chromatin-immunoprecipitation assay. Mutations within a predicted helix-turn-helix DNA-binding motif reduced binding and decreased mating efficiency. The deletion of four or more residues from the C-terminus of TraJ blocked its activity, but did not interfere with DNA binding. This feature, as well as homology to the C-terminal region of RovA and SlyA within the MarR/SlyA family, suggests that TraJ might counteract H-NS repression via a mechanism similar to these desilencing proteins.