Structure, expression and chromosomal localisation of the metallothionein-like gene family of tomato

Metallothioneins are small cysteine-rich proteins with strong binding capacity for heavy metals. In animals and fungi they are involved in cellular detoxification processes. Although genes for similar proteins exist in plants, less is known about the putative functions of their protein products. Here, we describe the characterisation of cDNAs specific for four genes (LEMT1, LEMT2, LEMT3 and LEMT4) encoding metallothionein-like proteins from tomato. Based on the characteristic cysteine pattern, the LEMT1, LEMT3 and LEMT4 gene products represent type 2 proteins. In contrast, the LEMT2 protein might establish a new structural pattern of metallothionein-like proteins not described before. Mapping experiments demonstrate that all four genes are localised at different genetic loci within the tomato genome. The members of the small gene family show a differential organ specific expression pattern. Expression of these genes is also influenced by heavy metals and by treatment with the thiol-oxidising drug diamide. We further describe the expression of the LEMT genes under different iron supply conditions both in tomato wild type as well as in the mutant chloronerva, which is defective in metal uptake regulation and exhibits a characteristic 'apparent iron deficiency syndrome'.