Operon Structure in Gram-Negative Bacteria

After the transport system genes, the merA gene encodes the large subunits of the dimeric protein mercuric reductase. Schiering et al. (1991) solved the crystal structure for the mercuric reductase protein from the Bacillus mer operon (see Sect. B.I.3). The structure (Fig. 2), as anticipated, is very similar to that of the homologous human protein glutathione reductase. The dimer subunits are related by a twofold rotational axis, and 

An additional feature in Fig. 2 worth noting is the amino-terminal 160 amino acids of mercuric reductase that lacked a fixed position in the crystal and therefore were not part of the solved structure. These contain the sequence that is homologous to MerP and postulated to be a mercurybinding domain. This region is drawn in Fig. 2 as an extension from the protein perhaps it functions like a baseball mitt that catches Hg from the membrane transport proteins and delivers Hg to the carboxyl-terminal catalytic binding site, so that, as in the bucket brigade model above, Hg is never found free within the cell. Mutant strains with the transport system but lacking the MerA detoxification enzyme are hypersensitive to mercury salts, as they accumulate Hg but cannot get rid of it. After reduction by NADH (via FAD and the active site cysteine pair), metallic Hg is released 

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