Expression of Rice Metallothionein Isoforms in Escherichia coli Enhances the Accumulation of Trivalent and Hexavalent Chromium

Introduction: Metallothioneins (MTs) are members of a family of low molecular weight and cysteine-rich proteins that are involved in heavy metal homeostasis and detoxification in living organisms. Plants have multiple MT types that are generally divided into four subgroups according to the arrangement of Cys residues. Methods: In the present study the E. coli cells which heterologously express four different rice MT (OsMT) isoforms were analyzed for the accumulation of two forms of chromium, Cr3+ and Cr6+. Results: The results show that the transgenic bacteria were more tolerant than control cells when they were grown up in the medium comprising Cr(NO3)3.9H2O or Na2CrO4. The cells expressing OsMT1, OsMT2, OsMT3 and OsMT4 give rise to 6.5-, 2.7-, 5.5- and 2.1-fold improvements on the accumulation capacity for Cr3+ and 9-, 3-, 5- and 3- fold Cr6+ respectively compared with comparison to the control strain. Furthermore, the purified recombinant GST-OsMTs were tested for their binding ability to Cr+3 and Cr+6 in vitro. Discussion: The data show that the recombinant GST-OsMT1 and GST-OsMT2 were able to bind both Cr3+ and Cr6+, in vitro. However, their binding strength was low with respect to previous tested divalent ions like Cd2+.