Molecular cloning of metallothionein cDNA and analysis of metallothionein gene expression in winter flounder tissues

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Abstract
Investigations into the precise role played by metallothionein (MT) in heavy-metal metabolism have been hampered by difficulties in positively identifying and quantifying MT in fish tissues. This study describes the development of an antisense MT RNA (cRNA) probe that will enable MT mRNA levels to be measured with a high degree of specificity and precision. Cadmium chloride administration induces the producton of MT mRNA in the liver and kidney of winter flounder (Pseudopleuronectes americanus). Poly(A)+ RNA purified from liver samples of winter flounder after cadmium chloride injections was used to construct a cDNA library. Several recombinant clones made complementary to MT mRNA were selected from this cDNA library by an oligonucleotide derived from the N-terminal amino acid sequence of winter flounder metallothionein. Sequence analysis of two of the cDNA inserts gave the structure of the entire 3′ untranslated region, a coding region corresponding to winter flounder MT and 49 nucleotides of the 5′ untranslated region. One of the flounder MT cDNAs, pWFMTC4, was subcloned into a RNA probe plasmid and transcribed to produce antisense MT RNA (cRNA). The MT cRNA was then used to detect the induction of MT mRNA production in the liver of winter flounder, following the administration of Cu2+, Zn2+, Cd2+, Pb2+, and Hg2+. The time required for the induction of hepatic MT mRNA by a single injection of Cd2+ was approximately 96 h. Dexamethasone did not induce an increase of MT mRNA in any of the winter flounder tissues examined (liver, kidney, heart, brain, intestinal scrape, and gill filament), whereas Cd2+ induced MT mRNA in all of the tissues except brain, where the constitutive level of expression was high.