Tripartite interaction between mercury resistance, thiol metabolism, and oxidative stress in bacteria

A student using a pipette. — Microbial Biology Ph.D. student Javiera Norambuena-Morales performing assays with *T. thermophilus. ?*

Keywords: Mercury resistance (mer) operon, sulfhydrylase, oxidateve stress response, Thermus thermophilus

Mercury resistance and the oxidtive stress response, which prevents intracellular oxygen damage, share dependence on the availability of cysteine. We are investigating how the supply of thiolated amino acids affects resistance to mercury and to reactive oxygen species (ROS) with the goal of learning how metals and oxidative stresses interact intracellularly.

Colonies of T. thermophilus. — Colonies of *T. thermophilus* that were mutated by the insertion of a kanamycin resistance gene to modify thiol biosynthesis and oxidative stress response.

Octopus Spring. — Octopus Spring in Yellowstone National Park is an example for a high temperature environment where *Thermus thermophilus* thrives

Thermus picture. — Most cells produce toxic reactive oxygen species (ROS in the figure), i.e., oxidative stress, when they respire. These species are quenched by small molecule thiol-based redox buffers and are detoxified by enzymes such as superoxide dismutase (SOD). Redox balance is maintained by the thioredoxin system (TR and Trx) that maintains intracellular redox.
When mercury (Hg) is not present in the cell environment, the Hg resistance (mer) operon is partially repressed expressing only MerR, the repressor of the operon, and Oah2, an enzyme involved in redox buffer biosynthesis. MerA, the enzyme that reduces and detoxifies Hg, is not made due to a tight repression of the merA promoter by MerR.
When Hg enters the cells, it is rapidly sequestered by the small molecule redox buffers such as bacilithiol (BSH). This challenges the thiol homeostasis, destabilizing the intracellular redox state leading to a higher requirement for electrons and thus increased respiration rates and oxidative stress. However, Hg also increases expression (thick lines) of the mer operon. This induction leads to production of more Oah2 and thus more BSH and of MerA and the detoxification of Hg by reduction to the volatile elemental form.