Silencing and activating type IV secretion genes of the F-like conjugative resistance plasmid R1.

Expression of DNA transfer (tra) genes of F-type conjugative plasmids is required for the assembly of a functional type IV secretion machinery and subsequent plasmid DNA transfer from donor to recipient cells. Transcription of tra genes depends on the activation of a single promoter, designated PY, by the plasmid encoded TraJ protein. We here determine plasmid specificity of TraJ proteins from various subgroups of F-like plasmids and find that plasmid R1 conjugation and PY promoter activation can be achieved only by its cognate activator and by TraJ of the Salmonella plasmid pSLT and not by F or R100 TraJ proteins. In addition, we characterize the PY promoter of plasmid R1. We show that TraJ binds to PY DNA in vivo and that H-NS acts as a silencer of the PY promoter. In the natural plasmid context, H-NS silences transfer gene expression and horizontal plasmid DNA transfer. In contrast to what was found for the F plasmid, lack of H-NS did not abolish the requirement for ArcA and TraJ to reach full tra gene expression and DNA transfer activity. We propose that, besides a passive de-silencing activity, both ArcA and TraJ play a direct role in synergistically stimulating tra operon transcription and subsequent DNA transfer.