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

A R KRAPP, MV HUMBERT AND N. CARRILLO

MICROBIOLOGY-UK

SOC GENERAL MICROBIOLOGY

Lugar: London; Año: 2011 vol. 157 p. 957 - 965

1350-0872

The soxRS regulon protects Escherichia coli cells against superoxide and nitric oxide. Oxidationof the SoxR sensor, a [2Fe?2S]-containing transcriptional regulator, triggers the response, but thenature of the cellular signal sensed by SoxR is still a matter of debate. In vivo, the sensor ismaintained in a reduced, inactive state by the activities of SoxR reductases, which employNADPH as an electron donor. The hypothesis that NADPH levels affect deployment of the soxRSresponse was tested by transforming E. coli cells with genes encoding enzymes and proteins thatlead to either build-up or depletion of the cellular NADPH pool. Introduction of NADP+-reducingenzymes, such as wheat non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase orE. coli malic enzyme, led to NADPH accumulation, inhibition of the soxRS regulon and enhancedsensitivity to the superoxide propagator methyl viologen (MV). Conversely, expression of peaferredoxin (Fd), a redox shuttle that can oxidize NADPH via ferredoxin-NADP(H) reductase,resulted in execution of the soxRS response in the absence of oxidative stress, and in highertolerance to MV. Processes that caused NADPH decline, including oxidative stress and Fdactivity, correlated with an increase in total (NADP++NADPH) stocks. SoxS expression can beinduced by Fd expression or by MV in anaerobiosis, under conditions in which NADPH is oxidizedbut no superoxide can be formed. The results indicate that activation of the soxRS regulon inE. coli cells exposed to superoxide-propagating compounds can be triggered by depletion of theNADPH stock rather than accumulation of superoxide itself. They also suggest that bacteria needto finely regulate homeostasis of the NADP(H) pool to enable proper deployment of this defensiveresponse.