Analysis of BnMTL, a novel metallothionein‐like protein in the bast fiber crop Ramie (Boehmeria nivea)

Ramie (Boehmeria nivea) is a perennial herb that is highly tolerant of heavy metals. In the present study, we cloned a novel metallothionein‐like gene from ramie; this gene, termed BnMTL, encodes a putative 46 amino acid protein with a molecular mass of 4.38 kDa. Analysis using quantitative RT‐PCR revealed that cadmium (Cd(2+)) treatment results in elevated expression of BnMTL in the roots. We heterologously overexpressed BnMTL in Escherichia coli cells to examine its binding to Cd(2+) and its possible role in homeostasis. Recombinant E. coli cells expressing BnMTL exhibited a high tolerance of Cd(2+) stress up to a concentration of 1 mm, and the observed accumulation of Cd(2+) was almost eight‐fold higher than the control. These results demonstrate that BnMTL (i) is highly expressed in the root following exposure to Cd(2+) and (ii) encodes a typical metallothionein‐like protein with high cadmium‐binding activity.