Rns, a virulence regulator within the AraC family, requires binding sites upstream and downstream of its own promoter to function as an activator

Abstract

Strains of enterotoxigenic Escherichia coli that express CS1 and CS2 pili require the transcriptional activator Rns, a member of the AraC family, for the expression of the pilin genes. Rns is also an activator of its own expression. However, the arrangement of its binding sites near its own promoter is unusual for a prokaryotic activator. Most activators have at least one binding site 30-80 nucleotides upstream of the transcription start site, but Rns has a single upstream binding site centred at -227. Rns also has two binding sites downstream of the transcription start site centred at +43 and +82, a region generally thought to be reserved for repressors. In vitro, the binding of a MBP::Rns fusion protein to each of these sites facilitates the binding of RNA polymerase to the rns promoter and the formation of an open complex. In vivo, the upstream binding site and one downstream site are required for Rns-dependent activation of its promoter despite the atypical location of these binding sites for an activator. This suggests that Rns may represent a new class of prokaryotic activators.