RESUMO
We report the sequence of the Mto gene, one of the two known metallothionein genes of Drosophila melanogaster, and compare its structure with that of the other metallothionein gene, Mtn. The main structural features are the presence of a small intron (61 base-pairs), the presence of four potential MREs (metal regulatory elements) and the absence of a TATA box in the promoter region. Of all metals tested, Hg2+, Cd2+ and Cu2+ are the most efficient ions for inducing an increase in Mto gene transcription. The Mto and Mtn genes are differentially regulated during normal development. Transcription of Mto is detected early in embryogenesis (0 to 3 h) and persists to the third larval instar, while Mtn expression starts later in embryogenesis (12 to 15 h) and is thereafter maintained throughout larval development and adult stages. Sequencing of the Mto protein is in good agreement with the nucleic acid data. Surprisingly, attempts to isolate and characterize the Mtn protein were unsuccessful. Several lines of evidence suggest that this metallothionein is rapidly incorporated after its synthesis into lysosomes, where it would be processed in a way that would not permit its purification. The function of the Mtn protein thus appears to be mainly related to detoxification processes. The pattern of expression of Mto suggests that this gene may be involved in the control of metal homeostasis during development.
Assuntos
Drosophila melanogaster/genética , Metalotioneína/genética , Sequência de Aminoácidos , Animais , Sequência de Bases , Mapeamento Cromossômico , Drosophila melanogaster/crescimento & desenvolvimento , Regulação da Expressão Gênica/efeitos dos fármacos , Genes , Íntrons , Metais/farmacologia , Dados de Sequência Molecular , Polimorfismo de Fragmento de Restrição , Sequências Reguladoras de Ácido Nucleico , Mapeamento por Restrição , Transcrição GênicaRESUMO
Metallothioneins (MT) are expressed during early development in species of several groups. To understand MT function in developmental processes, we studied the MT system of Xenopus laevis, a model vertebrate species in experimental embryology. We first purified and sequenced the liver MT from copper-treated animals. This 62-amino-acid protein shares the main structural properties of known vertebrate MT, and is more closely related to avian than to fish or mammalian MT. Using this sequence, we designed oligonucleotide primers to amplify and isolate a MT clone from a XL2 cell line cDNA library. This 752-bp cDNA encodes a putative 62-amino-acid-long protein that is 100% identical with the sequenced MT. Zinc, cadmium, and copper ions are very efficient inducers of MT mRNA accumulation in Xenopus liver and cell lines.