Heterologous Expression of a Rice Metallothionein type 1 Isoform in Esherichia coli and Study of It's Binding Ability to Nickel

Plants process several potential cellular mechanisms for detoxification of heavy metals. One of the most important mechanisms is synthesis of metal binding peptides and proteins such as metallothioneins (MTs). MTs are low molecular weight and cystein-rich proteins that can bind metal ions through their thiol groups. In this study the coding sequence of gene encoding OsMTI-1b isoforms from rice (), was cloned in pET41a and transferred into expression host, Escherichia coli strain Rosetta¬ (DE3). After induction with IPTG, considerable amount of recombinant proteins was produced in the soluble fraction of the E.coli transforman. Recombinant proteins were purified using affinity chromatography. The tolerance of cells expressing recombinant proteins toward Ni, were compared to control by plotting their growth curve in addition to determination of the amount of accumulated Ni ions in bacterial cells and culture medium using inductively coupled plasma atomic emission spectroscopy (ICP-AES). According to the results, over-expression of OsMTI-1b isoform increased the tolerance of E. coli cells to Ni through accumulation of more metal ions inside cells. Furthermore, the UV absorption spectra and competitive reactions of in vitro Ni-incubated proteins with 5-5' dithiobis (2-nitrobenzoic) acid (DTNB) revealed that GST-OsMTI-1b protein is able to form Ni-thiolate clusters. Taken together, these data indicate that OsMTI-1b isoform may be involved in protection of rice cells against heavy metal toxicity.