A bacterial hemerythrin-like protein MsmHr inhibits the SigF-dependent hydrogen peroxide response in mycobacteria

Hydrogen peroxide (H₂O₂) is one of a variety of reactive oxygen species (ROS) produced by aerobic organisms. Host production of toxic (H₂O₂) in response to pathogen infection is an important classical innate defense mechanism against invading microbes. Understanding the mechanisms by which pathogens, in response to oxidative stress, mediate defense against toxic ROS, can reveal anti-microbial targets and shed light on pathogenic mechanisms. In this study, we provide evidence that a Mycobacterium smegmatis hemerythrin-like protein MSMEG_2415, designated MsmHr, is a (H₂O₂)-modulated repressor of the SigF-mediated response to (H₂O₂). Circular dichroism and spectrophotometric analysis of MsmHr revealed properties characteristic of a typical hemerythrin-like protein. An msmHr knockout strain of M. smegmatis mc² 155 (δmsmHr) was more resistant to (H₂O₂) than its parental strain, and overexpression of MsmHr increased mycobacterial susceptibility to (H₂O₂). Mutagenesis studies revealed that the hemerythrin domain of MsmHr is required for the regulation of the (H₂O₂) response observed in the overexpression study. We show that MsmHr inhibits the expression of SigF (MSMEG_1804), an alternative sigma factor that plays an important role in bacterial oxidative stress responses, including those elicited by (H₂O₂), thus providing a mechanistic link between δmsmHr and its enhanced resistance to (H₂O₂). Together, these results strongly suggest that MsmHr is involved in the response of mycobacteria to (H₂O₂) by negatively regulating a sigma factor, a function not previously described for hemerythrins.