The mer operon of the acidophilic bacterium Thiobacillus T3.2 diverges from its Thiobacillus ferrooxidans counterpart

The chromosomal mercury resistance (mer) region of the acidophilic bacteriu m Thiobacillus T3.2 was cloned, characterized, and compared to reported hom ologous sequences. The Thiobacillus T3.2 mer resistance system is organized as an operon that transcribes into a polycistronic mRNA encoding the Hg2ion transport MerT and MerP proteins and the mercuric reductase MerA. In co ntrast to the Thiobacillus ferrooxidans mer determinant, no merC gene was d etected. Transcription of structural genes is regulated by the product of t he regulatory merR gene. On the basis of sequence data and expression exper iments in E. coli, both merTPA and merR transcription units could be locate d close to each other and in different strands, with their promoters (P-TPA and P-R, respectively) overlapping the putative MerR binding site in the i ntergenic operator/promoter (O/P) region. Amino acid sequences of mer gene products were compared to their homologs. Some sequence features, such as t he number and position of cysteine residues, are unique for the Mer protein s of this bacterium. Similarities (-10 and -35 boxes are 19 bp apart in bot h P-R and P-TPA promoters) and differences (inverted repeats in the Thiobac illus T3.2 MerR-binding site are 2 bp shorter than in Thiobacillus ferrooxi dans) exist between the O/P intergenic regions of both Thiobacilli. In vivo experiments showed inducible expression of mercury resistance in E. coli c ells transformed with the entire Thiobacillus T3.2 met genetic determinant (structural plus regulatory genes), and little or no expression in clones c ontaining only the structural merT, merP, and merA genes.