Molecular cloning and sequencing of metallothionein in squamates: new insights into the evolution of the metallothionein genes in vertebrates.

Metallothioneins are cysteine-rich, metal-binding proteins ubiquitously expressed in living organisms. In the last past years, a plethora of vertebrate metallothionein sequences have become available, but so far there has been an almost absolute lack of data about sequences of metallothionein of non-avian diapsida. In the framework of the investigations on structural and functional properties of non-mammalian metallothioneins, we have cloned and sequenced the cDNAs encoding for metallothioneins of 10 squamate reptiles, belonging to 5 different infraorders. These sequences have been used to gain insight into the evolutionary history of metallothioneins in reptiles. Phylogenetic analysis shows that reptilian metallothionein phylogeny is inconsistent with the species phylogeny. Such findings allow us to hypothesize that the identified metallothionein in each squamate species used for this study might be considered a paralogous gene derived from more events of gene duplication and losses occurred during the diversification of the squamate species. Finally, through vertebrate metallothionein comparisons and phylogenetic analysis, we also add a novel contribution to the understanding of the evolution of metallothionein genes along the major vertebrate lineages.