Plant Metallothioneins: Putative Functions Identified by Promoter Analysis in Silico

Metallothioneins are low-molecular-weight proteins capable of covalently binding heavy metal ions due to the presence of many cysteine residues in their sequences. We analyzed the predicted amino acid sequences of 19 metallothionein (7 from Arabidopsis thaliana and 12 from Oryza sativa) and their promoter sequences in silico in order to determine the potential regulatory cis-elements present in the promoters of metallothionein genes, from which it is possible to determine the putative functions of these genes. The PlantCARE and PLACE databases provided information about the putative regulatory elements in the metallothionein promoters. Metal response element sequences were found in the promoters of eleven O. sativa and two Arabidopsis metallothionein genes. Copper response elements were identified in both model plants, usually in many copies, particularly in O. sativa. Both the high cysteine content and the presence of metal response motifs in the promoters support the suggestion that metallothioneins play a key role in metal detoxification. The most common putative element in the analyzed promoters was CIRCADIAN, which was present in five A. thaliana and eight O. sativa sequences. The methyl jasmonate response sequence, root-specific expression element and drought response element were found only in O. sativa metallothioneins. Light and low temperature response elements, biotic and abiotic stress elements, an abscisic acid-responsive element and an ethylene-responsive element occur in selected metallothionein promoters of both species. A few promoters have putative organand cell-specific regulatory elements. The presence of many different motifs in the promoters of the Arabidopsis and O. sativa genes implies that metallothioneins are general stress response proteins with many important functions in plants, including regulation of their normal development and adaptation to changing environmental conditions.