Recombinant tilapia (Oreochromis mossambicus) fish metallothionein (MT) was used as a surface biosorbent for mercury removal in Escherichia coli. Fish MT conferred better resistance than did mouse or human MT. When tilapia MT (tMT) was fused with an outer-membrane protein, outer membrane protein C (OmpC), the membrane-targeted fusion protein, OmpC-tMT, gave enhanced resistance compared with cytoplasmic tMT expressed in the same host cell. The cytoplasmically expressed tMT showed high mercury adsorption (4.3 +/- 0.4 mg/g cell dry weight). The cell surface that expressed E. coli showed about 25% higher adsorption ability (5.6 +/- 0.4 mg/g) than the cells expressing cytoplasmic MT, attaining almost twice the level of adsorption of the control plasmid (3.0 +/- 0.4 mg/g). As MTs are also known for their ability to scavenge hydroxyl-free radicals, it was also shown that tMT exhibited better radical-scavenging activities than glutathione. These results suggest that fish MT has potential for the development of a bioremediation system for mercury removal that protects the harboring E. coli host by free-radical scavenging.
Cysteine-rich metallothioneins (MTs) have been reported to possess the capacity to scavenge reactive oxygen species in vitro and in vivo. Recombinant strains of Escherichia coli expressing outer membrane protein C (OmpC) fused with MTs from human, mouse and tilapia displayed the ability for such surface-localized MTs to scavenge extracellular free radicals, but the benefits of the possible applications of this capacity have not yet been demonstrated. Because the intrinsic butanol tolerance of microbes has become an impediment for biological butanol production, we examined whether surface-displayed MTs could contribute to butanol tolerance. The results show that strains expressing OmpC-MT fusion proteins had higher butanol tolerance than strains with cytoplasmically expressed MTs. Furthermore, the OmpC-tilapia MT fusion protein enhanced butanol tolerance more strongly than other recombinant constructs. Although the enhanced level of tolerance was not as high as that provided by OmpC-tilapia MT, over-expression of OmpC was also found to contribute to butanol tolerance. These results suggest that free-radical scavenging by MT and OmpC-related osmoregulation enhance butanol tolerance. Our results shed new light on methods for engineering bacteria with higher butanol tolerance.
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