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Journal of Bacteriology, January 2001, p. 162-170, Vol. 183, No. 1
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.1.162-170.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Control of the Ferric Citrate Transport System of Escherichia coli: Mutations in Region 2.1 of the FecI Extracytoplasmic-Function Sigma Factor Suppress Mutations in the FecR Transmembrane Regulatory Protein

Alfred Stiefel, Susanne Mahren, Martina Ochs, Petra T. Schindler, Sabine Enz, and Volkmar Braun*

Mikrobiologie/Membranphysiologie, Universität Tübingen, 72076 Tübingen, Germany

Received 26 July 2000/Accepted 13 October 2000

Transcription of the ferric citrate transport genes is initiated by binding of ferric citrate to the FecA protein in the outer membrane of Escherichia coli K-12. Bound ferric citrate does not have to be transported but initiates a signal that is transmitted by FecA across the outer membrane and by FecR across the cytoplasmic membrane into the cytoplasm, where the FecI extracytoplasmic-function (ECF) sigma factor becomes active. In this study, we isolated transcription initiation-negative missense mutants in the cytoplasmic region of FecR that were located at four sites, L13Q, W19R, W39R, and W50R, which are highly conserved in FecR-like open reading frames of the Pseudomonas aeruginosa, Pseudomonas putida, Bordetella pertussis, Bordetella bronchiseptica, and Caulobacter crescentus genomes. The cytoplasmic portion of the FecR mutant proteins, FecR1-85, did not interact with wild-type FecI, in contrast to wild-type FecR1-85, which induced FecI-mediated fecB transport gene transcription. Two missense mutations in region 2.1 of FecI, S15A and H20E, partially restored induction of ferric citrate transport gene induction of the fecR mutants by ferric citrate. Region 2.1 of sigma 70 is thought to bind RNA polymerase core enzyme; the residual activity of mutated FecI in the absence of FecR, however, was not higher than that of wild-type FecI. In addition, missense mutations in the fecI promoter region resulted in a twofold increased transcription in fecR wild-type cells and a partial restoration of fec transport gene transcription in the fecR mutants. The mutations reduced binding of the Fe2+ Fur repressor and as a consequence enhanced fecI transcription. The data reveal properties of the FecI ECF factor distinct from those of sigma 70 and further support the novel transcription initiation model in which the cytoplasmic portion of FecR is important for FecI activity.

* Corresponding author. Mailing address: Mikrobiologie/Membranphysiologie, Universität Tübingen, Auf der Morgenstelle 28, D-72074 Tübingen, Germany. Phone: 49-7071-2972096. Fax: 49-7071-295843. E-mail: volkmar.braun{at}mikrobio.uni-tuebingen.de.

Journal of Bacteriology, January 2001, p. 162-170, Vol. 183, No. 1
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.1.162-170.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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