Site-directed mutagenesis of HgcA and HgcB reveals amino acid residues important for mercury methylation.
Methylmercury is a potent neurotoxin that is produced by anaerobic microorganisms from inorganic mercury by a recently discovered pathway. A two-gene cluster, consisting of hgcA and hgcB, encodes two of the proteins essential for this activity. hgcA encodes a corrinoid protein with a strictly conserved cysteine proposed to be the ligand for cobalt in the corrinoid cofactor, whereas hgcB encodes a ferredoxin-like protein thought to be an electron donor to HgcA. Deletion of either gene eliminates mercury methylation by the methylator Desulfovibrio desulfuricans ND132. Here, site-directed mutants of HgcA and HgcB were constructed to determine amino acid residues essential for mercury methylation. Mutations of the strictly conserved residue Cys93 in HgcA, the proposed ligand for the corrinoid cobalt, to Ala or Thr completely abolished the methylation capacity, but a His substitution produced measurable methylmercury. Mutations of conserved amino acids near Cys93 had various impacts on the methylation capacity but showed that the structure of the putative "cap helix" region harboring Cys93 is crucial for methylation function. In the ferredoxin-like protein HgcB, only one of two conserved cysteines found at the C terminus was necessary for methylation, but either cysteine sufficed. An additional, strictly conserved cysteine, Cys73, was also determined to be essential for methylation. This study supports the previously predicted importance of Cys93 in HgcA for methylation of mercury and reveals additional residues in HgcA and HgcB that facilitate the production of this neurotoxin.
Biochemistry Division, University of Missouri, Columbia, Missouri, USA.
Steven D
SD
Smith
Biochemistry Division, University of Missouri, Columbia, Missouri, USA.
Romain
R
Bridou
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Alexander
A
Johs
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Jerry M
JM
Parks
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee, USA.
Dwayne A
DA
Elias
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Richard A
RA
Hurt
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee, USA.
Steven D
SD
Brown
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Department of Microbiology, University of Tennessee, Knoxville, Tennessee, USA.
Mircea
M
Podar
Biochemistry Division, University of Missouri, Columbia, Missouri, USA Molecular Microbiology and Immunology, University of Missouri, Columbia, Missouri, USA wallj@missouri.edu.
Judy D
JD
Wall
D000596:Amino Acids / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence; D004252:DNA Mutational Analysis; D045285:Desulfovibrio desulfuricans / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D008628:Mercury / Q000378:metabolism*; D008767:Methylmercury Compounds / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism
D000596:Amino Acids; D001426:Bacterial Proteins; D008767:Methylmercury Compounds; D050505:Mutant Proteins; D008628:Mercury