Sonneratia alba (S. alba) is a mangrove species grown in brackish water of tropical and subtropical regions. Due to its unique environment, it has evolved various mechanisms for modulating salt and metal levels. In order to find the genes connected with bioaccumulation of metals, the root transcriptome annotation of Sonneratia alba was analyzed and a new metallothionein (MT) gene was cloned. Sequence analysis found that the new MT gene belongs to type 3 MT, which is mostly expressed in roots. A simple and efficient method was used to express the type 3 MT of S. alba (SaMT3) by transforming the recombinant expression vector pET15b-SaMT3 into Escherichia coli (E. coli) Rosetta-gami and induction with the optimal conditions of 500 μM Isopropyl β-D-1-thiogalactopyranoside (IPTG) at 24ºC for 12 h. OD₆₀₀ of E. coli cells expressing His fused SaMT3 protein after treated with 500 μM Cu²⁺ or 500 μM Pb²⁺ for 12 h can reach 1.01 or 0.98, while OD₆₀₀ of control cells expressing His-tag can reach only 0.81 or 0.75. Both control cells and the cells expressing SaMT3 accumulated metals. Cells expressing SaMT3, however, accumulated more Pb²⁺ and Cu²⁺ (more than two times) than control cells. In vivo, real-time PCR showed that the SaMT3 transcript was induced significantly when stimulated with 250 μM, 500 μM, or 1,000 μM Cu²⁺ or Pb²⁺ for 24 h and 48 h. Taken together, the expression of SaMT3 can increase Cu²⁺ and Pb²⁺ resistance and binding capacity of E. coli.
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