A metallothionein-like gene htMT2 strongly expressed in internodes and nodes of Helianthus tuberosus and effects of metal ion treatment on its expression

Abstract

A cDNA sequence encoding a type-2 metallothionein (MT)-like protein, designated htMT2, was isolated from a Helianthus tuberosus L. tuber cDNA library. The isolated cDNA is 509 bp, coding a 7.8-kDa polypeptide. Two partial genomic fragments covering the open reading frame of htMT2 were cloned by PCR. The fragments htMTG-1 (986 bp) and htMTG-2 (982 bp) contain three exons and two introns. The N- and C-terminal domains of the predicted polypeptide have eight and seven cysteine residues, separated by a central cysteine-free spacer. Sequence alignment revealed that the predicted protein was homologous to type-2 MTs of plants. Southern blot analysis indicated that htMT2 is encoded by a small multi-gene family in the H. tuberosus genome. Northern blot analysis showed that htMT2 transcripts were predominantly expressed in internodes and nodes, but were low in leaves, leafstalks, tubers and young roots, and none was detected in roots. Treatment with Cu²⁺ reduced the expression of htMT2 in internodes, nodes, leaves and leafstalks. In addition, the expression levels in internodes and nodes share an inverse relationship with the concentrations of Cu²⁺. In internodes and nodes, treatment with Zn²⁺ at 10 and 100 μM reduced the expression levels of htMT2, and 1,000 μM Zn²⁺reduced it to the lowest level, but 500 μM Zn²⁺ had little effect. The expressions of htMT2 in different tissues were not appreciably affected by heat shock. It is suggested that HtMT2 might be involved in the transport or availability of Cu²⁺ and Zn²⁺ to some apo-metal enzymes or apo-metal proteins.