The genetic basis for bacterial mercury methylation

Area of Science:

• Environmental Microbiology
• Biogeochemistry
• Toxicology

Background:

• Methylmercury is a potent neurotoxin formed from inorganic mercury by anaerobic bacteria.
• The specific genes and proteins involved in this mercury methylation process have remained largely unknown.
• Understanding this pathway is critical for environmental and health risk assessments.

Purpose of the Study:

• To identify the genes and proteins responsible for mercury methylation in bacteria.
• To elucidate the genetic basis of methylmercury production in natural environments.
• To investigate the conservation of the mercury methylation pathway across different microbial species.

Main Methods:

• Genetic analysis involving gene deletion experiments in Desulfovibrio desulfuricans ND132 and Geobacter sulfurreducens PCA.
• Bioinformatic analysis of sequenced genomes to identify homologous genes in other bacteria and archaea.
• Biochemical characterization of the identified gene products, HgcA and HgcB.

Main Results:

• A two-gene cluster, hgcA and hgcB, was identified as essential for mercury methylation in both studied bacterial species.
• Deletion of either hgcA or hgcB abolished mercury methylation, confirming their critical roles.
• HgcA was identified as a putative corrinoid protein (methyl carrier), and HgcB as a 2[4Fe-4S] ferredoxin (electron donor).
• Homologs of hgcA and hgcB were found in known mercury-methylating bacteria and archaea but not in non-methylators, suggesting a conserved pathway.

Conclusions:

• The hgcA and hgcB genes constitute a core genetic element for mercury methylation in bacteria and archaea.
• The identified proteins HgcA and HgcB provide insights into the biochemical mechanism of mercury methylation.
• This discovery suggests a common evolutionary origin for mercury methylation across diverse microbial lineages and has implications for understanding global mercury cycling.