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.