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The FecI Extracytoplasmic-Function Sigma Factor of Escherichia coli Interacts with the ß' Subunit of RNA Polymerase

Mikrobiologie/Membranphysiologie, Universität Tübingen, D-72076 Tübingen, Germany

Received 21 October 2002/ Accepted 12 December 2002

Transcription of the ferric citrate transport system of Escherichia coli K-12 is mediated by the extracytoplasmic-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 ß' subunit of RNA polymerase. The inactive mutant protein FecI(K155E) displayed reduced binding to ß', and small deletions along the entire FecI protein led to total impairment of ß' binding. In vitro, FecI was retained on Ni²⁺-nitrilotriacetic acid agarose loaded with a His-tagged ß'_1-313 fragment and coeluted with ß'_1-313. Binding of FecI to ß' and ß'_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 ß' subunit of RNA polymerase and of binding being enhanced by the protein that activates the ECF sigma factor.

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