Evolutionary Mode of Metallothioneins Inferred from Cd-MT Genes of Tetrahymena

From Tetrahymena thermophila （strain BF5）, the coding region of Cd-MT gene was cloned and sequenced. and identified as MTT1 isoform. A serial duplicate structure is discovered in its amino acid sequence, which separates the coding region into three parts （Part 1：7-61; Part 2：64-118; Part 3：122-162）. The alignments among them and comparison with the corresponding parts of MT1 isoform suggest that MT1 and MTT1 isoforms both come from the same ancient gene that is homologous to Part 1, and Cd-MTs of Tetrahymena are aroused by such ancient gene duplication. The prediction of secondary structures and the analysis of the disulfide-bonding state of cysteine show that there are a lot of differences between MT1 and MTT1 isoforms, which maybe relate to their function mechanism.

Molecular Mechanisms and Genetic Basis of Heavy Metal Tolerance/ Hyperaccumulation in Plants

Phytoremediation has gained increased attention as a cost-effective method for the remediation of heavy metal-contaminated sites. Because some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration, which is manifested by an interaction between a genotype and its environment. The growing application of molecular genetic technologies has led to increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance, as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. In the present review, our major objective is to concisely evaluate the progress made so far in understanding the molecular/cellular mechanisms and genetic basis that control the uptake and detoxification of metals by plants.

Functional Characterization of a Type 4 Metallothionein Gene(CsMT4) in Cucumber

The excess amounts of heavy metals are toxic to various physiological processes in plants.Plant metallothioneins(MTs)are low molecular weight,cysteine-rich metal-binding proteins that play important roles in the detoxification of heavy metal ions.In this study,we characterized Cs MT4,a plant type 4 MT gene identified in a complementary DNA(cDNA)library prepared from young cucumber(Cucumis sativus)fruit.Cs MT4encodes a 90 amino acid protein with a predicted molecular mass of 9.028 kD.CsMT4 contains 17 cysteine residues in three highly conserved cysteine-rich domains.In contrast to the structures of other MTs,the highly conserved amino acid pattern CxCxxxCxCxxCxC is present in the middle of CsMT4.Furthermore,CsMT4 was markedly induced in various tissues by various concentrations of Cd^(2+)and Zn^(2+)(>1.0 mmol·L^(-1)).Similar to AtMT4b function,heterologous expression of Cs MT4 in E.coli could also improved its tolerance to Cd^(2+)and led to increased uptake of Cd^(2+),and the rate of Cd^(2+)uptake was the highest in cells expressing a phytochelatin-like peptide.Our findings demonstrate that CsMT4 might be obviously induced by the metal stress in cucumber,and improves tolerance to Cd ions but not Zn ions when heterologously expressed in E.coli,and suggest that the composition and arrangement of N-terminal Cys-residues in MT4 are associated with their binding capacity and preference for different metal ions.