TY - JOUR TI - Mercury toxicity and detoxification in Thermus thermophilus HB27 DO - https://doi.org/doi:10.7282/t3-xnfc-2t42 PY - 2018 AB - Mercury (Hg) is one of the most toxic and widely distributed heavy metals. To detoxify this metal the mercury (mer) resistance operon is present some Bacteria and Archaea. This is the most studied mechanism of Hg-detoxification. This dissertation describes how the mer operon is regulated in Thermus thermophilus HB27, how two of the genes present in this operon (oah2 and merR) are involved in Hg(II) resistance, as well as low molecular weight (LMW) thiol and reactive oxygen species (ROS) responsive systems. T. thermophilus HB27 is a Gram-negative thermophile that has a very peculiar mer operon, it consists of merA (mercuric reductase), an hypothetical protein (hp), merR (regulator), and oah2, which encodes for an enzyme that synthesizes homocysteine. Therefore, the mer operon in T. thermophilus HB27 links mercury resistance to low-molecular weight (LMW) thiols biosynthesis. To determine the role of each gene in Hg-detoxification, mutant strains were constructed and their response to Hg was analyzed. I found that the mer operon has two promoters, one appears to be independent of regulation by MerR and in the other, MerR mediates response to Hg(II) as a repressor/activator. It was also determined that oah2 as well as other LMW thiols biosynthetic genes (oah1, bshA, bshB and oas) and the thioredoxin system are involved in Hg resistance. I discovered that bacillithiol (BSH) is the major LMW thiol in strain HB27 and showed that Hg(II) caused depletion of the reduced BSH pool. This depletion was associated with an increase in ROS upon Hg(II) exposure and free iron concentration in the cytoplasm. I showed that ROS were triggered by Hg(II) and that superoxide dismutase and pseudocatalase, both known ROS detoxifying enzymes that maintain intracellular redox state, are involved in Hg(II) resistance. These results suggest that small thiols play a role in T. thermophilus HB27’s response to Hg stress, possibly providing a buffer for Hg that is later removed by MerA. If thiols are oxidized by Hg(II), oxidative stress is produced leading to an increase in ROS. Collectively, the research presented in this dissertation describes how Hg(II) regulates the mer operon, interacts with LMW thiols and produces ROS in the extremophile T. thermophilus. Studying the physiology of T. thermophilus provides clues about the origin and evolution of mechanisms for mercury resistance and toxicity, as we as the oxidative stress response. KW - Microbial Biology KW - Mercury--Toxicology LA - eng ER -