Characterization and expression of DNA sequences encoding putative type-IImetallothioneins in the seagrass Posidonia oceanica

Posidonia oceanica is a marine phanerogam, largely widespread in the Medite rranean sea, representing an important food substrate fur many marine organ isms. A progressive reduction of P. oceanica meadows has been reported, due to anthropogenic coastal activity. Studying mechanisms by which this speci es responds to environmental stresses, three DNA sequences putatively encod ing metallothioneins (MTs) have been isolated, by PCR. Two sequences, Pomt2 a (accession no. AT249603) and Pomt2b (accession no. AJ249602), show high s imilarities with genes encoding type-II MTs and are interrupted by two and one intron, respectively. The third sequence, Pomt2c (accession no. AJ24960 4), is supposed to be a pseudogene, originated by retrotranscription of the Pomt2b mRNA. These sequences belong to a multigene family with at least fi ve members. Northern hybridizations indicated that MT transcripts accumulat ion is constitutive and seasonally regulated. MT encoding RNAs increase aft er rhyzome harvesting and (at a lesser extent) after 15 d of cultivation in an aquarium, As for animal MTs, transcripts accumulation is observed also after exposure to trace metals such as copper and cadmium. In the case of c opper, the effect depends on concentration. Finally, taking into considerat ion the great interest in studying the biogeochemical cycle of mercury in t he Mediterranean basin and since P. oceanica is commonly considered a bioin dicator of this metal, the effect of mercury treatments on the accumulation of MT transcripts has been analyzed: in only a few experiments a small inc rease in the level of transcripts was recorded, suggesting that MTs are not key elements in the mercury accumulation by this species.