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Characterization of Transcriptional Regulation of Shewanella frigidimarina Fe(III)-Induced Flavocytochrome c Reveals a Novel Iron-Responsive Gene Regulation System

Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ,¹ School of Biological Sciences, University of Essex, Wivenhoe Park, Essex,² Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom³

Received 21 February 2003/ Accepted 16 April 2003

The bacterium Shewanella frigidimarina can grow anaerobically by utilizing Fe(III) as a respiratory electron acceptor. This results in the synthesis of a number of periplasmic c-type cytochromes, which are absent when the organism is grown in the absence of added Fe(III). One cytochrome, IfcA, is synthesized when Fe(III) is present as the sole respiratory electron acceptor or when it is present in combination with oxygen, fumarate, or nitrate. The ifcA gene was thus selected for a study of iron-responsive gene regulation of respiratory proteins in S. frigidimarina. The monocistronic ifcA gene clusters with two other monocistronic genes, ifcO, encoding a putative outer membrane porin, and ifcR, encoding a putative transcriptional regulator of the LysR superfamily. Analysis of transcription of all three genes under a range of growth conditions in the wild type and an ifcR insertion mutant and analysis of a strain that constitutively expresses ifcR revealed that iron regulation is exerted at the level of ifcR transcription. In the presence of Fe(III) IfcR is synthesized and acts positively to regulate expression of ifcO and ifcA. Control of Fe(III) respiration by this novel regulatory system differs markedly from Fur-mediated regulation of iron assimilation, in which Fur serves as an Fe(II)-activated repressor.

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