A Novel Mercuric Reductase from the Unique Deep Brine Environment of Atlantis II in the Red Sea▪▪This article contains supplemental Figs. 1–3, Tables 1 and 2, and additional references

• Published in: The Journal of biological chemistry 2014-01, Vol.289 (3), p.1675-1687
• Main Authors: Sayed, Ahmed, Ghazy, Mohamed A., Ferreira, Ari J.S., Setubal, João C., Chambergo, Felipe S., Ouf, Amged, Adel, Mustafa, Dawe, Adam S., Archer, John A.C., Bajic, Vladimir B., Siam, Rania, El-Dorry, Hamza
• Format: Article
• Language: English
• Subjects: Atlantis II Brine Pool; Enzyme Kinetics; Enzyme Mechanisms; Enzyme Structure; Extreme Halophilic; Mercuric Reductase; Metagenomics; Mutagenesis Site-specific; Red Sea Atlantis II; Thermophilic
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M113.493429

A unique combination of physicochemical conditions prevails in the lower convective layer (LCL) of the brine pool at Atlantis II (ATII) Deep in the Red Sea. With a maximum depth of over 2000 m, the pool is characterized by acidic pH (5.3), high temperature (68 °C), salinity (26%), low light levels, anoxia, and high concentrations of heavy metals. We have established a metagenomic dataset derived from the microbial community in the LCL, and here we describe a gene for a novel mercuric reductase, a key component of the bacterial detoxification system for mercuric and organomercurial species. The metagenome-derived gene and an ortholog from an uncultured soil bacterium were synthesized and expressed in Escherichia coli. The properties of their products show that, in contrast to the soil enzyme, the ATII-LCL mercuric reductase is functional in high salt, stable at high temperatures, resistant to high concentrations of Hg2+, and efficiently detoxifies Hg2+in vivo. Interestingly, despite the marked functional differences between the orthologs, their amino acid sequences differ by less than 10%. Site-directed mutagenesis and kinetic analysis of the mutant enzymes, in conjunction with three-dimensional modeling, have identified distinct structural features that contribute to extreme halophilicity, thermostability, and high detoxification capacity, suggesting that these were acquired independently during the evolution of this enzyme. Thus, our work provides fundamental structural insights into a novel protein that has undergone multiple biochemical and biophysical adaptations to promote the survival of microorganisms that reside in the extremely demanding environment of the ATII-LCL. Background: Molecular features underlying enzyme function in extreme environments are poorly understood. Results: Identification of the basis for thermostability, halophilicity, and detoxification activity in a mercuric reductase from hot deep-sea brine. Conclusion: A small number of structural modifications accounts for the enzyme's robustness. Significance: This work defines novel adaptations that enable enzymes to cope with multiple abiotic stressors simultaneously.