Acute Toxicity of Divalent Mercury to Bacteria Explained by the Formation of Dicysteinate and Tetracysteinate Complexes Bound to Proteins in Escherichia coli and Bacillus subtilis

Bacteria are the most abundant organisms
on Earth and also the
major life form affected by mercury (Hg) poisoning in aquatic and
terrestrial food webs. In this study, we applied high energy-resolution
X-ray absorption near edge structure (HR-XANES) spectroscopy to bacteria
with intracellular concentrations of Hg as low as 0.7 ng/mg (ppm)
for identifying the intracellular molecular forms and trafficking
pathways of Hg in bacteria at environmentally relevant concentrations.
Gram-positive Bacillus subtilis and
Gram-negative Escherichia coli were
exposed to three Hg species: HgCl2, Hg–dicysteinate
(Hg­(Cys)2), and Hg–dithioglycolate (Hg­(TGA)2). In all cases, Hg was transformed into new two- and four-coordinate
cysteinate complexes, interpreted to be bound, respectively, to the
consensus metal-binding CXXC motif and zinc finger domains of proteins,
with glutathione acting as a transfer ligand. Replacement of zinc
cofactors essential to gene regulatory proteins with Hg would inhibit
vital functions such as DNA transcription and repair and is suggested
to be a main cause of Hg genotoxicity.