The isolation and characterisation of type II metallothionein-like genes from tomato (Lycopersicon esculentum L)

Two cDNAs encoding putative metallothionein (MT)-like peptides have been isolated from tomato (L. esculentum L.). The predicted protein products of these two cDNAs (LeMT(A) and LeMT(B)) are 72 and 83 amino acids respectively and both encode peptides with arrangements of cysteine residues characteristic of type II plant MTs. In other plants which possess more than one gene expressing MT proteins of the same type, the products are closely related or identical, but LeMT(A) and LeMT(B) constitute two different classes of message, and encode two different protein products. Northern blot analysis of LeMT(A) and LeMT(B) showed that transcripts of both MT-like genes were more abundant in leaves than roots in tomato plants grown without addition of extra metal ions, a characteristic of type II MTs. A genomic clone corresponding to LeMT(B) (LeMT(B)) was isolated and sequenced. The 5'-flanking region of LeMT(B) was shown to contain a putative metal regulatory element (MRE) which suggests the possibility of metal-regulated transcription. In addition, the upstream region also contains a G-box like motif (CACGTG) and an 8 bp sequence (AATTCAAA) found within the promoters of genes shown to be ethylene-responsive.