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)nGly (EC8 (n = 8), EC11 (n = 11), and EC20 (n = 20)) were synthesized, linked to a lpp‐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. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 70: 518–524, 2000.