Metallothioneins are involved in detoxification of heavy metals. A cDNA encoding type 3 metallothionein (PcMT3)
was isolated from the salt stressed leaf cDNA library of Porteresia coarctata (Roxb.) Tateoka (wild rice) that grows
well in the heavy metal laden estuarine soils. The PcMT3 cDNA (581 bp) encodes a protein of 64 amino acids. PcMT3
is highly homologous (82 %) to OsMT-I-3a of rice, but is unique from other type 3 plant MTs due to the presence of an
additional glycine residue in the C-terminal domain. Analysis of the 5′ upstream region of PcMT3 showed the presence
of cis-acting elements like the CG box and STRE previously reported to be involved in gene expression under heavy
metal stress. Southern analysis suggested the presence of more than one copy of PcMT3-like sequences in the
P. coarctata genome. Analysis of genomic clone of PcMT3 revealed the presence of two introns. A comparison of the
genomic sequence of PcMT3 with closely similar type 3 MTs from rice and mangrove species revealed conservation in
the number and position of introns. Transcript profiling for PcMT3 in P. coarctata leaves in the presence of Cd, Cu and
Zn showed an increase in transcript accumulation.