Enhanced bioaccumulation of heavy metals by bacterial cells displaying synthetic phytochelatins

A novel strategy using synthetic phytochelatins is described for the purpose of developing microbial agents for enhanced bioaccumulation of toxic metals. Synthetic genes encoding for several metal-chelating phytochelatin analogs (Glu-Cys)(n)Gly (EC8 (n = 8), EC11 (n = 11), and EC20 (n = 20)) were synthesized, linked to a Ipp-ompA fusion gene, and displayed on the surface of E. coli. For comparison, EC20 was also expressed periplasmically as a fusion with the maltose-binding protein (MBP-EC20). Purified MBP-EC20 was shown to accumulate more Cd2+ per peptide than typical mammalian metallothioneins with a stoichiometry of 10 Cd2+/peptide. Cells displaying synthetic phytochelatins exhibited chain-length dependent increase in metal accumulation. For example, 18 nmoles of Cd2+/mg dry cells were accumulated by cells displaying EC8, whereas cells exhibiting EC20 accumulated a maximum of 60 nmoles of Cd2+/mg dry cells. Moreover, cells with surface-expressed EC20 accumulated twice the amount of Cd2+ as cells expressing EC20 periplasmically. The ability to genetically engineer ECs with precisely defined chain length could provide an attractive strategy for developing high-affinity bioadsorbents suitable for heavy metal removal. (C) 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 70: 518-524, 2000.