This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Manganese contributes to anti-oxidative stress particularly in catalase-devoid bacteria, and DtxR family metalloregulators, through sensing cellular Mn content, regulate its homeostasis. Here, we show that metalloregulator MntR (So-MntR) functions dually as Mn and HO sensors in mediating HO resistance by an oral streptococcus. HO disrupted So-MntR binding to Mn transporter promoter and induced disulfide-linked dimerization of the protein. Mass spectrometry identified Cys-11/Cys-156 and Cys-11/Cys-11 disulfide-linked peptides in HO-treated So-MntR. Site mutagenesis of Cys-11 and Cys-156 and particularly Cys-11 abolished HO-induced disulfide-linked dimers and weakened HO damage on So-MntR binding, indicating that HO inactivates So-MntR via disulfide-linked dimerization. So-MntR C123S mutant was extremely sensitive to HO oxidization in dimerization/oligomerization, probably because the mutagenesis caused a conformational change that facilitates Cys-11/Cys-156 disulfide linkage. Intermolecular Cys-11/Cys-11 disulfide was detected in C123S/C156S double mutant. Redox Western blot detected So-MntR oligomers in air-exposed cells but remarkably decreased upon HO pulsing, suggesting a proteolysis of the disulfide-linked So-MntR oligomers. Remarkably, elevated C11S and C156S but much lower C123S proteins were detected in HO-pulsed cells, confirming Cys-11 and Cys-156 contributed to HO-induced oligomerization and degradation. Accordingly, in the C11S and C156S mutants, expression of and cellular Mn decreased, but HO susceptibility increased. In the C123S mutant, increased expression, cellular Mn content, and manganese-mediated HO survival were determined. Given the wide distribution of Cys-11 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.

Edited by Ursula JakobAquaporins (AQPs) are transmembrane proteins widely distributed in various organisms, and they facilitate bidirectional diffusion of water and uncharged solutes. The catalase-negative bacterium Streptococcus oligofermentans produces the highest H 2 O 2 levels reported to date, which has to be exported to avoid oxidative stress. Here, we report that a S. oligofermentans aquaporin functions as a peroxiporin facilitating bidirectional transmembrane H 2 O 2 transport. Knockout of this aquaporin homolog, So-AqpA, reduced H 2 O 2 export by ϳ50% and increased endogenous H 2 O 2 retention, as indicated by the cellular H 2 O 2 reporter HyPer. Heterologous expression of So-aqpA accelerated exogenous H 2 O 2 influx into Saccharomyces cerevisiae and Escherichia coli cells, indicating that So-AqpA acts as an H 2 O 2transferring aquaporin. Alanine substitution revealed Phe-40 as a key residue for So-AqpA-mediated H 2 O 2 transport. Northern blotting, qPCR, and luciferase reporter assays disclosed that H 2 O 2 induces a >10-fold expression of So-aqpA. Super-resolution imaging showed that H 2 O 2 treatment increases So-AqpA protein molecules per cell by 1.6-to 3-fold. Inactivation of two redox-regulatory transcriptional repressors, PerR and MntR, reduced H 2 O 2 -induced So-aqpA expression to 1.8-and 4-fold, respectively. Electrophoretic mobility shift assays determined that MntR, but not PerR, binds to the So-aqpA promoter, indicating that MntR directly regulates H 2 O 2 -induced So-aqpA expression. Importantly, So-aqpA deletion decreased oxic growth and intraspecies competition and diminished the competitive advantages of S. oligofermentans over the caries pathogen Streptococcus mutans. Of note, So-aqpA orthologs with the functionally important Phe-40 are present in all streptococci. Our work has uncovered an intrinsic, H 2 O 2 -inducible bacterial peroxiporin that has a key physiological role in H 2 O 2 detoxification in S. oligofermentans.