A. Sayed, M.A. Ghazy, A.J. Ferreira, J.C. Setubal, F.S. Chambergo, A. Ouf, M. Adel, A.S. Dawe, J.A. Archer,V.B. Bajic, R. Siam, H. El-Dorry
J Biol Chem., 289(3), 1675-87, (2014)
Atlantis II Brine Pool, Enzyme Kinetics, Enzyme Mechanisms, Enzyme Structure, Extreme Halophilic, Mercuric Reductase, Metagenomics, Mutagenesis Site-specific, Red Sea Atlantis II, Thermophilic
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 (68oC) and 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 elemental mercury. The metagenome-derived gene and an ortholog from an uncultured soil bacterium were synthesized and expressed in E. 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 temperature, 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 3D modeling, have identified distinct structural features that contribute to extreme halophilicity, thermostability and high detoxification capacity respectively, 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.