Crystal structures of the F and pSLT plasmid TraJ N-terminal regions reveal similar homodimeric PAS folds with functional interchangeability.

In the F family of conjugative plasmids, TraJ is an essential transcriptional activator of the tra operon that encodes most of the proteins required for conjugation. Here we report for the first time the X-ray crystal structures of the TraJ N-terminal domains from the prototypic F plasmid (TraJF(11-130)) and from the Salmonella virulence plasmid pSLT (TraJpSLT(1-128)). Both structures contain similar Per-ARNT-Sim (PAS) folds, which further homodimerize through the N-terminal helix and the structurally conserved β-sheet of the PAS fold from each protomer. Mutational analysis reveals that the observed dimeric interface is critical for TraJF transcriptional activation, indicating that dimerization of TraJ is required for its in vivo function. TraJ is specific in activating its cognate tra operon promoter; however, heterologous PAS domains from pSLT and R100 TraJ can functionally replace the TraJF PAS domain, suggesting that the allelic specificity of TraJ is solely mediated by the region C-terminal to the PAS domain.