**6. Proposed mechanism for metallothionein induction by metal cations**

Cellular effects of exposure to the divalent metal cations and roles of particular resistance mechanism of the metallothionein expression, and expression of stress protein GroEL is shown in the **Figure 3**. GroEL forms essential component which reduces heavy metal stress [22]. The MTs is a cysteine rich protein which binds with the metals and thereby detoxified through reducing its presense [23]. The heavy metals enter the cells by the transport of the proteins which binds the metals. This metals could further binds SH groups with repressor protein of smtB. These attached metals further alter conforms the repressor protein and then released it. These process permits RNA polymerase to initiate the transcription from smtA, resulted the metallothionein expression.

#### **Figure 3.**

*Proposed mechanism for metallothionein induction by metal cations and arsenic oxyanions. (1) divalent metal cation challenge from the environment, (2) sulfhydryl (-SH) modification by metal or arsenate binding, (3) SmtA expression producing metallothionein proteins, (4) metal sequestration and detoxification by metallothionein proteins.*

### **7. Plant metallothioneins**

After the Phytochelatin discovery, as a key metal ligand essential for the plant tolerance to Cadmium, a MT protein was identified in the wheat, and numerous MT genes was isolated from the plants. The plant Class II MT proteins are further classified based upon the amino acid sequence. In the year 1993, Robinson and his co-workers identified the two plant based MT which was based on cysteine residue positions in the predicted proteins. Since then, number of characterized plant MT genes has been increased drastically, as many do not conform to these 2 groups, some additional categories are added [24, 25].

The majority of the plant MT genes are identified in angiosperms. Numerous species, like *Arabidopsis*, sugarcane and rice contain some genes encoding all 4 MTs types. This shows that the evolution of 4 plant MT types predates the separation of the monocots and dicots, and are likely that the major flowering plants also contains 4 different MT types. The presence of 4 MTs types in the plants with a distinct arrangement of the cysteines contrasts with situation in animals. For an example, the 4 mouse MTs all contain the same conserved cysteines, although they do differ in tissue expression [17]. The diversity of plant MT gene family suggests that these might differ not only in the sequence but also in the function. There is only little information about the MT genes in the non-flowering plant species. However, the genes encoding the Type3 MTs are cloned from different gymnosperms. One of the MT-encoding gene are isolated from a brown alga, *Fucus vesiculocus*, [26]. This MT do not fit readily into any of the 4 plant types described above but, primarily relies on basis of cysteine residues, is equally similar to the Arabidopsis MT1a and an oyster MT.
