Structure, organization and expression of the metallothionein gene family inArabidopsis

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Recent results indicate that plants also possess functional metallothionein genes. Here we report the cloning and characterization of five metallothionein genes fromArabidopsis thaliana. The position of the single intron in each gene is conserved. The proteins encoded by these genes can be divided into two groups (MT1 and MT2) based on the presence or absence of a central domain separating two cysteine-rich domains. Four of the MT genes (MT1a,MT1c,MT2a andMT2b) are transcribed inArabidopsis. Several lines of evidence suggest that the fifth gene,MT1b, is inactive. There is differential regulation of the MT gene family. MT1 mRNA is expressed highly in roots, moderately in leaves and is barely detected in inflorescences and siliques. MT2a and MT2b mRNAs are more abundant in leaves, inflorescences and in roots from mature plants, but are also detected in roots of young plants, and in siliques. MT2a mRNA is strongly induced in seedlings by CUSO₄, whereas MT2b mRNA is relatively abundant in this tissue and levels increase only slightly upon exposure to copper.MT1a andMT1c are located within 2 kb of each other and have been mapped to chromosome 1.MT1b andMT2b map to separate loci on chromosome V, andMT2a is located on chromosome III. The locations of these MT genes are different from that ofCAD1, a gene involved in cadmium tolerance inArabidopsis.