ChrR, a soluble quinone reductase of Pseudomonas putida that defends against H2O2

Most bacteria contain soluble quinone-reducing flavoenzymes. However, no biological benefit for this activity has previously been demonstrated. ChrR of Pseudomonas putida is one such enzyme that has also been characterized as a chromate reductase; yet we propose that it is the quinone-reducing activity of ChrR that has the greatest biological significance. ChrR reduces quinones by simultaneous two-electron transfer, avoiding formation of highly reactive semiquinone intermediates and producing quinols that promote tolerance of H2O2. Expression of chrR was induced by H2O2, and levels of chrR expression in overexpressing, wild type, and knock-out mutant strains correlated with the H2O2 tolerance and scavenging ability of each strain. The chrR expression level also correlated with intracellular H2O2 levels as measured by protein carbonylation assays and fluorescence-activated cell scanning analysis with the H2O2-responsive dye H(2)DCFDA. Thus, enhancing the activity of ChrR in a chromate-remediating bacterial strain may not only increase the rate of chromate transformation, it may also augment the capacity of these cells to withstand the unavoidable production of H2O2 that accompanies chromate reduction.