Manganese (Mn²⁺) is essential for organisms in the resistance of oxidative stresses, particularly in catalase-devoid streptococci. The DtxR family metalloregulators, through sensing cellular Mn²⁺ content, regulate its homeostasis and avoid the cellular toxicity of excess manganese ions. However, how these DtxR-like metalloregulators regulate Mn²⁺ import by sensing cellular redox status remains unknown.

Prof. DONG Xiuzhu’s group at the State Key Laboratory of Microbial Resources, Institute of Microbiology of Chinese Academy of Sciences, clarifies for the first time the redox regulatory mechanism of a DtxR-like metalloregulator MntR in streptococcus, and reveals MntR functioning as a H₂O₂ sensor in addition to metalloregulator.

Metalloregulators are unique to prokaryotes. The novel function in controlling anti-oxidative stresses in bacteria revealed in this work would shed light on pathogenic bacteria control or drug designing. The paper has been published online in The Journal of Biological Chemistry. 

The metalloregulator MntR is previously believed as a Mn²⁺ sensor. In this study, researchers found that MntR (So-MntR) also functions as a H₂O₂ sensor and mediates H₂O₂ resistance in an oral streptococcus (see the Figure below).

Biochemistry analysis revealed that H₂O₂ disrupted the association between So-MntR and the Mn²⁺ transporter mntABC by inducing disulfide-linked dimerization of So-MntR. Site mutagenesis of the key cysteine residues reduced H₂O₂ damage on So-MntR as well as the protein-DNA association, therefore confirming cysteines being the H₂O₂-oxidative targets. 

H₂O₂ damage on So-MntR was further verified in vivo. Redox Western blot detected So-MntR oligomers in air-exposed cells, but remarkably decreased upon H₂O₂ pulsing. The key cysteine residues mutagenesis abolished So-MntR degradation under H₂O₂ stress, confirming that these cysteines contribute to H₂O₂-induced oligomerization and degradation of MntR. 

The physiological experiments indicated that key cysteine mutagenesis led to decreased mntABC expression, reduced cellular Mn²⁺ content, and Mn-mediated H₂O₂ survival, thereby supporting that So-MntR functions as a H₂O₂ sensor and mediates oxidant defense by controlling Mn²⁺ transport.

Given the wide distribution of Cys11 in streptococcal DtxR-like metalloregulators, the disclosed redox regulatory function and mechanism of So-MntR can be employed by the DtxR family proteins in bacterial resistance to oxidative stress. 

Redox-switches based on the redox-sensitive cysteine thiol groups have been widely utilized by H₂O₂ sensors in conducting the regulatory functions in response to ROS. The traditional metalloregulator functions as a H₂O₂ sensor found in this study sheds light on that any protein, including regulatory proteins, carrying redox reactive thiol groups in cysteine or methionine residues can be involved in the regulation of redox reactions. 

This work is supported by National Natural Science Foundation of China.