Ramadan E.Maged M.Hosseiny A.E.Chambergo F.S.Setubal J.C.Dorry H.E.Department of BiologySchool of Sciences and EngineeringThe American University in CairoNew CairoEgypt; Escola de Artes Ci�ncias e HumanidadesUniversidade de S�o PauloS�o PauloBrazil; Instituto de Qu�micaUniversidade de S�o PauloS�o PauloBrazil; Faculty of PharmacyDepartment of Pharmacology and BiochemistryThe British University in EgyptEl-Sherouk CityEgypt; Faculty of BiotechnologyOctober University for Modern Sciences and ArtsCairoEgypt2020-01-092020-01-092019992240https://doi.org/10.1128/AEM.01431-18PubMedID30504211https://t.ly/8pOP0ScopusThe hypersaline Kebrit Deep brine pool in the Red Sea is characterized by high levels of toxic heavy metals. Here, we describe two structurally related mercuric reductases (MerAs) from this site which were expressed in Escherichia coli. Sequence similarities suggest that both genes are derived from proteobacteria, most likely the Betaproteobacteria or Gammaproteobacteria. We show that one of the enzymes (K35NH) is strongly inhibited by NaCl, while the other (K09H) is activated in a NaCl-dependent manner. We infer from this difference that the two forms might support the detoxification of mercury in bacterial microorganisms that employ the compatible solutes and salt-in strategies, respectively. Three-dimensional structure modeling shows that all amino acid substitutions unique to each type are located outside the domain responsible for formation of the active MerA homodimer, and the vast majority of these are found on the surface of the molecule. Moreover, K09H exhibits the predominance of acidic over hydrophobic side chains that is typical of halophilic salt-dependent proteins. These findings enhance our understanding of how selection pressures imposed by two environmental stressors have endowed MerA enzymes with catalytic properties that can potentially function in microorganisms that utilize distinct mechanisms for osmotic balance in hypersaline environments. � 2019 American Society for Microbiology. All Rights Reserved.EnglishAtlantis II DeepKebrit DeepMercuric reductaseRed Sea brine poolsDetoxificationEnzymesEscherichia coliHeavy metalsAmino acid substitutionAtlantis ii deepsEnvironmental stressorsHypersaline environmentKebrit DeepMercuric reductaseRed seaThree dimensional structure modelingSodium chlorideamino acidbrinecoliform bacteriumdetoxificationenvironmental stressenzymeenzyme activityheavy metalhypersaline environmentmolecular analysisproteinIndian OceanRed Sea [Indian Ocean]Bacteria (microorganisms)BetaproteobacteriaEscherichia coliGammaproteobacteriaProteobacteriaArticlehttps://doi.org/10.1128/AEM.01431-18PubMedID30504211