Primary sequence of duck metallothionein

Metallothionein cDNA, promoter, and genomic sequences of the tropical green mussel, Perna viridis

The primary structure of the cDNA and metallothionein (MT) genomic sequences of the tropical green mussel (Perna viridis) was determined. The complete cDNA sequences were obtained using degenerate primers designed from known metallothionein consensus amino acid sequences from the temperate species Mytilus edulis. The amino acid sequences of P. viridis metallothionein deduced from the coding region consisted of 72 amino acids with 21 cysteine residues and 9 Cys-X-Cys motifs corresponding to Type I MT class of other species. Two different genomic sequences coding for the same mRNA were obtained. Each putative gene contained a unique 5'UTR and two unique introns located at the same splice sites. The promoters for both genes were different in length and both contained metal responsive elements and active protein-binding sites. The structures of the genomic clones were compared with those of other species. J. Exp. Zool. 284:445-453, 1999.

Multiple isoforms of metallothionein are expressed in the porcine liver

Isogenes are highly homologous to each other and are often difficult to ascertain, as has been the case with metallothionein, a metal-binding protein rich in cysteines. Conventional separation of metallothionein isoforms relied on ion exchange chromatography of the proteins, or screening for the sequences from gene libraries. In this study, a combination of RT-PCR and partial protein sequencing is used in the identification of metallothionein isogenes expressed in porcine liver. By this approach, we have identified expressed coding sequences which constitute 10 new isogenes. Of the four known groups of metallothioneins (MT), phylogenetic analyses place these pig isogenes in the MT-1 group, except two which are identified as being closely related to MT-2, and none in groups 3 and 4. The isogenes are thus namedpMT-1a to -1g, andpMT-2a and -2b. While each of the isogene sequences is unique, two isogenes,pMT-1e1 andpMT-1e2, share an identical amino acid sequence, differing only in specific codons. Two others,pMT-1b andpMT-1g, have a cysteine substituted by arginine, the first such sequence ever detected in MT.pMT-2a andpMT-2b are closely aligned with the MT-2 group of vertebrates, in spite of the absence of a characteristic acidic amino acid at position 10 or 11, common in other mammalian metallothioneins.

Structure and expression of metallothionein gene in ducks

Metallothionein (MT) cDNAs were cloned and sequenced from two genera of ducks, Muscovy (Cairina muschata) and Tsai ya (Anas platyrhynchos). The two cDNAs show an extremely high sequence homology and contain an open reading frame encoding 63 amino acids. MT mRNA expressions were studied after metal induction using the cloned cDNA as a probe. Cadmium and copper induced MT gene efficiently, whereas zinc showed a markedly less effect. In addition, the MT mRNA accumulations in various developmental stages were also investigated. The result reveals a different pattern of expression from that of mammals. The discrepancy in MT gene between Tsai ya and Muscovy was further explored by examining genomic DNA structures. The duck MT showed three exons and two introns. The most significant variation of the genes occurs at intron II in which Tsai ya MT has 24 bases more than Muscovy MT. Moreover, MT expressions in the hybrids of Muscovy and Tsai ya were investigated using a reverse transcriptase-polymerase chain reaction. Those results demonstrated that parental MT genes are expressed in the hybrids after metal induction.

Functional integrity of metallothionein genes in testicular cell lines

The presence and inducibility of the major cadmium (Cd) chelating protein metallothionein (MT) in testicular cells has been controversial. In this study, the induction and production of MT in testicular cells were studied using mouse Leydig and Sertoli cell lines. Metal accumulation was studied by subjecting the cells to increasing levels of Cd. The presence of transcription factors for MT synthesis was analyzed by transfecting the cells with a reporter gene under the control of the MT promoter. The dose- and time-dependent induction of MT were conducted by Northern analyses. Expression of MT genes occurred in both Leydig and Sertoli cells. To avoid cross hybridization of the MT probe with mRNAs encoding testicular metal binding proteins and to investigate the integrity of MT mRNA, isoMT mRNA identification and primer extension experiments were performed. Those studies show that the induced mRNA indeed encodes MT. The biosynthesis of MT was confirmed by following 35S-cysteine incorporation into the protein. Finally, cadmium tolerance of testicular cells is compared with that of fibroblast cells. By these studies, we conclude that the MT genes are functional and inducible in testicular cells.

Purification and characterization of a cadmium-induced metallothionein from the shore crab Carcinus maenas (L.)

Two metallothionein variants were purified from the midgut gland of crabs (Carcinus maenas) exposed to a high cadmium concentration (2 p.p.m.). One of the variants was purified from crabs exposed to a low cadmium concentration (0.5 p.p.m.). The purification method involved acetone precipitation, gel filtration and reversed-phase h.p.l.c. The complete amino acid sequences of both variants have been elucidated by m.s. and automated sequence analysis on S-methylated proteins or fragments produced by cleavage of the S-methylated proteins with Staphylococcus aureus proteinase. The two variants from crabs exposed to the high cadmium concentration differed only by a single residue of methionine at the N-terminus. The single variant isolated from crabs exposed to the low cadmium concentration was the one without the N-terminal methionine, indicating that high cadmium concentrations either inhibit the processing enzymes and/or that the processing enzymes cannot keep pace with the increased metallothionein synthesis when cadmium availability is high. Cadmium-induced metallothionein from C. maenas shows a high degree of structural similarity to metallothioneins from the decapod crustaceans Scylla serrata and Homarus americanus.

Cloning and nucleotide sequence of a complementary DNA encoding Xenopus laevis metallothionein: mRNA accumulation in response to heavy metals

A cDNA encoding Xenopus laevis metallothionein (MT) was cloned from a cDNA library constructed using liver poly(A+)RNA of X. laevis adult males treated with CdCl2. The probe used to screen the library was a MT-specific DNA fragment obtained by means of the polymerase chain reaction (PCR) and degenerate oligodeoxynucleotide primers. The cDNA clone encodes a putative protein of 62 amino acids, of which 20 are cysteine residues. The position of all the cysteine residues is conserved with respect to mammalian MT sequences. The amino acid sequence of this X. laevis MT, designated XIMT-A, shares between 60% and 67% identity with various vertebrate MTs. Overall, the structure of XIMT-A is no similar in sequence to MT-1 than it is to MT-2 isoforms of various vertebrates. Ten different X. laevis MT cDNA isolates were partially sequenced and turned out to be identical, suggesting a single species of MT mRNA. Southern blot analysis of X. laevis DNA reveals that the XlMT-A gene is present in at least two copies. This result is consistent with the suggestion that a genome duplication occurred in a X. laevis ancestor. The in vivo response to increasing doses of Cd2+, Zn2+, and Cu2+ metal salts was tested. In the liver, all three metals proved to be potent inducers, raising MT mRNA levels between 50- and 100-fold. The maximum response to Cd2+ was at 12 hr after injection and to Zn2+ at 24 hr after injection. High levels of mRNA were maintained for more than 48 hr. Cd2+ and Zn2+ induced XlMT-A mRNA in all tissues examined (kidney, spleen, heart, intestine, testes, and brain). Dexamethasone did not induce MT mRNA synthesis in the liver.

Evolution of avian metallothionein: DNA sequence analyses of the turkey metallothionein gene and metallothionein cDNAs from pheasant and quail

The turkey metallothionein gene (tkMT) was isolated from a phage lambda-turkey genomic DNA library by virtue of high identity with chicken MT cDNA. The nucleotide sequences of the proximal 240 bp of the 5'-flanking region, of each of the three exons, and of the intron/exon boundaries were determined. Comparisons of the nucleotide sequences of the tkMT and cMT genes revealed (1) absolute conservation of intronic DNA immediately flanking each respective intron/exon boundary, (2) high conservation (95.6%) of exonic DNA encoding translated regions of the mRNA, and (3) high conservation (95%) of exonic DNA encompassing the putative transcription start point and polyadenylation signals. Sequence comparisons of the tkMT and cMT promoters regions near the TATA box revealed that both promoters contain a highly conserved proximal metal-responsive enhancer (MRE-enhancer) motif. The deduced amino acid sequence (63 amino acids) of tkMT was identical with that of cMT. In order to further explore the degree of conservation of the protein coding regions of avian MT genes, partial MT cDNAs from turkey, quail (qMT), and pheasant (pMT) were amplified using the reverse transcriptase-polymerase chain reaction (RT-PCR) and primers corresponding to the amino- and carboxyl-terminal coding regions of cMT mRNA. RT-PCR reaction products were cloned and the DNA sequences of multiple cDNA clones from each species were determined. The results suggest the existence of a single MT mRNA in zinc-treated liver from turkey and pheasant and the existence of a major and possibly a minor MT mRNA in quail.(ABSTRACT TRUNCATED AT 250 WORDS)