The FecI extracytoplasmic-function sigma factor of Escherichia coli interacts with the β′ subunit of RNA polymerase

Transcription of the ferric citrate transport system of Escherichia coli K-12 is mediated by the extracyto-plasmic-function (ECF) sigma factor FecI, which is activated by ferric citrate in the growth medium. By using a bacterial two-hybrid system, it was shown in vivo that FecI binds to the beta' subunit of RNA polymerase. The inactive mutant protein FecI(K155E) displayed reduced binding to beta', and small deletions along the entire FecI protein led to total impairment of beta' binding. In vitro, FecI was retained on Ni2+-nitrilotriacetic acid agarose loaded with a His-tagged beta'(1-313) fragment and coeluted with beta'(1-313). Binding of Fecl to beta' and beta'(1-313) was enhanced by FecR(1-85), which represents the cytoplasmic portion of the FecR protein that transmits the inducing signal across the cytoplasmic membrane. Interaction of FecR with FecI was demonstrated by showing that isolated FecR inhibited degradation of FecI by trypsin. This is the first demonstration of binding of an ECF sigma factor of the FecI type to the beta' subunit of RNA polymerase and of binding being enhanced by the protein that activates the ECF sigma factor.