Molecular connectivity between extra-cytoplasmic sigma factors and PhoP accounts for integrated mycobacterial stress response

The main purpose of this study is to understand how mycobacteria can sense numerous stress conditions and mount an appropriate stress response. Recent studies suggest that at low pH M. tuberculosis encounters reductive stress, and in response, modulates redox homeostasis by utilizing the phoPR regulatory system. However, the mechanism of integrated regulation of stress response remains unknown. To probe how PhoP contributes to redox stress response, we find that a PhoP-depleted M. tuberculosis shows a significantly enhanced susceptibility to redox stress relative to the WT bacilli. In keeping with these results, PhoP was shown to contribute to mycothiol redox state. Because SigH, one of the alternative sigma factors of mycobacteria, is known to control expression of redox inducible genes, we probed whether previously-reported PhoP-SigH interaction accounts for mycobacterial redox stress response. We had shown that under acidic conditions PhoP functions in maintaining pH homeostasis via its interaction with SigE. In striking contrast, here we show that under redox stress, direct recruitment of SigH, but not PhoP-SigH interaction, controls expression of mycobacterial thioredoxin genes, a major mycobacterial anti-oxidant system. Together, these unexpected results uncover novel stress-specific enhanced or reduced interaction events of sigma factors and PhoP, as the underlying mechanisms of an adaptive programme, which couples low pH conditions and mycobacterial thiol redox homeostasis. SignificanceM. tuberculosis encounters reductive stress under acidic pH. To investigate the mechanism of integrated stress response, we show that PhoP plays a major role in mycobacterial redox stress response. We observed a significant correlation between phoP-dependent and redox-active expression of thioredoxin genes, a major mycobacterial antioxidant system. Further probing on functioning of regulators reveals that while PhoP controls pH homeostasis via its interaction with SigE, direct recruitment of SigH, but not PhoP-SigH interaction, controls expression of thioredoxin genes. These strikingly contrasting results showing enhanced PhoP-SigE interaction under acidic pH and reduced PhoP-SigH interaction under redox conditions, uncover the underlying novel mechanism of mycobacterial adaptive program, coupling low pH with maintenance of redox homeostasis.