Mercuric reductase activity of multiple heavy metal-resistant Lysinibacillus sphaericus G1

A culture was isolated from an industrial mercuric salt-contaminated soil, which could tolerate Cd, Co, Zn, Cr, and Hg up to 190, 525, 350, 935, and 370 μM, respectively. The isolate was identified as Lysinibacillus sphaericus by 16S rRNA gene sequencing. It bioaccumulated Cd, Co, and Zn, and reductively detoxified Cr and Hg. Chromate reductase and mercuric reductase (MerA) activities in the cell extract were 2.4 and 0.13 units mg(-1) protein, respectively. The study also describes designing of broad-specificity primers based on firmicute merA genes. These primers were successfully used to amplify a 440 bp merA fragment from the current isolate. Based on the partial sequence, complete merA ORF of 1641 bp was amplified. It showed 99% similarity to a putative merA gene from distantly related Streptococcus agalactiae, but only 72% identity with the well-characterized merA from a more closely related Bacillus cereus RC607. The gene sequence possessed all the features required for the functioning of MerA enzyme, and its function was confirmed by recombinant expression in E. coli. To the best of our knowledge, this is the first report of full length merA gene from L. sphaericus.