The soxRS response of Escherichia coli can be induced in the absence of oxidative stress and oxygen by modulation of NADPH content

Abstract

ThesoxRSregulon protectsEscherichia colicells against superoxide and nitric oxide. Oxidation of the SoxR sensor, a [2Fe–2S]-containing transcriptional regulator, triggers the response, but the nature of the cellular signal sensed by SoxR is still a matter of debate.In vivo, the sensor is maintained in a reduced, inactive state by the activities of SoxR reductases, which employ NADPH as an electron donor. The hypothesis that NADPH levels affect deployment of thesoxRSresponse was tested by transformingE. colicells with genes encoding enzymes and proteins that lead to either build-up or depletion of the cellular NADPH pool. Introduction of NADP+-reducing enzymes, such as wheat non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase orE. colimalic enzyme, led to NADPH accumulation, inhibition of thesoxRSregulon and enhanced sensitivity to the superoxide propagator methyl viologen (MV). Conversely, expression of pea ferredoxin (Fd), a redox shuttle that can oxidize NADPH via ferredoxin-NADP(H) reductase, resulted in execution of thesoxRSresponse in the absence of oxidative stress, and in higher tolerance to MV. Processes that caused NADPH decline, including oxidative stress and Fd activity, correlated with an increase in total (NADP++NADPH) stocks. SoxS expression can be induced by Fd expression or by MV in anaerobiosis, under conditions in which NADPH is oxidized but no superoxide can be formed. The results indicate that activation of thesoxRSregulon inE. colicells exposed to superoxide-propagating compounds can be triggered by depletion of the NADPH stock rather than accumulation of superoxide itself. They also suggest that bacteria need to finely regulate homeostasis of the NADP(H) pool to enable proper deployment of this defensive response.