Molecular characterization of FinR, a novel redox-sensing transcriptional regulator in Pseudomonas putida KT2440

Abstract

FinR is required for the induction offpr(ferredoxin-NADP+reductase) under superoxide stress conditions inPseudomonas putida. Many proteobacteria harbour FinR homologues in their genome as a putative LysR-type protein. Three cysteine residues (at positions 150, 239 and 289 inP. putidaFinR) are conserved in all FinR homologues. When these conserved cysteines, along with two other cysteine residues present in FinR, were individually mutated to serines, the FinR remained active, unlike SoxR and OxyR inEscherichia coli. The results of ourin vitroDNA-binding assay with cellular extracts showed that FinR binds directly to thefprpromoter region. In order to identify the FinR functional domain for sensing superoxide stress, we employed random and site-directed mutagenesis of FinR. Among 18 single amino acid mutants, three mutants (T39A, R194A and E225A) abolishedfprinduction without any alteration of their DNA-binding ability, whereas other mutants also abrogated their DNA-binding abilities. Interestingly, two mutants (L215P and D51A) appeared to be constitutively active, regardless of superoxide stress conditions. Ferrous iron depletion, ferric iron addition andfdxA(ferredoxin) gene deletion also participate in the regulation offpr. These data indicate that FinR has unusual residues for redox sensing and that the redox-sensing mechanism of FinR differs from the well-known mechanisms of OxyR and SoxR.