Thermal Stability of a Mercuric Reductase from the Red Sea Atlantis II Hot Brine Environment as Analyzed by Site-Directed Mutagenesis.

The lower convective layer (LCL) of the Atlantis II brine pool of the Red Sea is a unique environment in terms of high salinity， temperature， and high concentrations of heavy metals. Mercuric reductase enzymes functional in such extreme conditions could be considered a potential tool in the environmental detoxification of mercurial poisoning and might alleviate ecological hazards in the mining industry. Here， we constructed a mercuric reductase library from Atlantis II， from which we identified genes encoding two thermostable mercuric reductase (MerA) isoforms: one is halophilic (designated ATII-LCL) while the other is not (designated ATII-LCL-NH). The ATII-LCL MerA has a short motif composed of four aspartic acids (4D414-417) and two characteristic signature boxes that played a crucial role in its thermal stability. To further understand the mechanism behind the thermostability of the two studied enzymes， we mutated the isoform ATII-LCL-NH and found that the substitution of 2 aspartic acids (2D) at positions 415 and 416 enhanced the thermal stability， while other mutations had the opposite effect. The 2D mutant showed superior thermal tolerance， as it retained 81% of its activity after 10 min of incubation at 70°C. A three-dimensional structure prediction revealed newly formed salt bridges and H bonds in the 2D mutant compared to the parent molecule. To the best of our knowledge， this study is the first to rationally design a mercuric reductase with enhanced thermal stability， which we propose to have a strong potential in the bioremediation of mercurial poisoning. The Red Sea is an attractive environment for bioprospecting. There are 25 brine-filled deeps in the Red Sea. The Atlantis II brine pool is the biggest and hottest of such hydrothermal ecosystems. We generated an environmental mercuric reductase library from the lowermost layer of the Atlantis II brine pool， in which we identified two variants of the mercuric reductase enzyme (MerA). One is the previously described halophilic and thermostable ATII-LCL MerA and the other is a nonhalophilic relatively less thermostable enzyme， designated ATII-LCL-NH MerA. We used the ATII-LCL-NH enzyme as a parent molecule to locate the amino acid residues involved in the noticeably higher thermotolerance of the homolog ATII-LCL MerA. Moreover， we designed a novel enzyme with superior thermal stability. This enzyme might have strong potential in the bioremediation of mercuric toxicity.