Analysis of type 1 metallothionein cDNAs in Vicia faba

In animals and fungi, small cysteine-rich proteins called metallothioneins (MTs) play a role in heavy metal tolerance. MT genes have been isolated in plants, but their function remains to be elucidated. We have isolated two distinct Vicia faba MT genes that belong to the type 1 group of plant MT genes in contrast to a MT gene we previously isolated that belongs to type 2. We found similarities and differences between the V. faba MT genes. The RNA expression patterns differed and this was most pronounced in roots, which contained high MT1 but very low MT2 RNA levels. Like MT2, MT1 transcript levels were not significantly affected by treatment with Cd, Cu, Fe and Zn, at least under the experimental conditions. MT RNA levels varied in leaves and stem internodes of different developmental ages, with the highest expression in the older tissue. The levels of MT RNA correlated inversely with endogenous Cd, Cu and Fe levels within different internodes, but not with a number of other metals tested (including Zn). The three bean MTs were expressed in Escherichia coli and found to bind Cd, Cu and Zn but not to Fe. The MTs were tested to determine if they differed in their ability to bind a specific metal but no significant differences in binding were observed.