Cadmium Metallothionein Genes of Ciliates

First cadmium metallothionein like gene (PMCd1) from a ciliate, Paramecium sp. and a novel cadmium inducible metallothionein (TMCd1) gene has been identified and sequenced from the locally isolated ciliate; Tetrahymena tropicalis lahorensis isolated from industrial wastewater. Both the genes (PMCd1 and TMCd1) are intronless, encoding 612 and 471 nucleotides respectively, with TGA as the stop codon and TAA coding for glutamine. The coding region of PMCd1 comprises 203 amino acids, including 37 cysteine residues with a conserved structural pattern arranged in 17 cys-x-y-cys, 1 cys-cys and cys contexts. The TMCd1 is quite different from the previously reported MT genes in T. pyriformis and T. pigmentosa. However, it shows 78% homology with four different Cd-MT genes reported from T. thermophila. The amino acids sequence of TMCd1 has a special feature of three CCCX6CCX6CCCX6CC and two CCX6CXCX2CXCC intragenic tandem repeats with a conserved structural pattern of cysteine. The translated protein of TMCd1 contains 30.12% cysteine residues, which is typical of Tetrahymena Cd inducible MT genes. A TATA box is located in the 5` flanking region at nucleotide 34-38 upstream region of ATG.