Molecular cloning and characterization of Ipomoea nil metallothioneins

Abstract

Metallothioneins are low-molecular-weight, cysteine-rich proteins able to bind a variety of heavy metal ions. They are involved in the maintenance of micronutrient homeostasis and detoxification of nonessential heavy metals. Plant metallothioneins are classified into four main types based on the number and arrangement of cysteine residues. Here we report on the isolation of novel complementary and genomic DNA from a model for a short-day plant, Ipomoea nil (Japanese morning glory), that encodes 66- or 76-amino acid residue proteins. In silico and phylogenetic analysis of putative amino acid sequences confirmed that these 66- and 76-amino acid residue proteins belong, respectively, to type 1 and type 2 plant metallothioneins. In genomic sequences of the analyzed metallothioneins, two introns were recognized, one of them located at an evolutionary conserved position. Furthermore, examination of the expression patterns of cloned metallothioneins in I. nil organs and in cotyledons in response to copper and zinc exposure suggested that their physiological functions could be diversified. Moreover, the observed changes of analyzed I. nil metallothioneins in response to heavy metal treatment indicate that InMT1 may be involved in copper homeostasis in I. nil cotyledons. The role of InMT2 in micronutrient homeostasis remains questionable.