GENE STRUCTURE, DIFFERENTIAL EXPRESSION AND PHYLOGENETIC ANALYSIS OF THE METALLOTHIONEIN GENE FAMILY OF THE SEA URCHIN PARACENTROTUS LIVIDUS

Metallothioneins (MT) are small and cysteine-rich proteins that bind metal ions such as zinc, copper, cadmium, and nickel1. Several functions were proposed for these peptides, ranging from toxic metal protection to physiological metal homeostasis, free radical scavenging, oxidative stress protection, antiapoptotic defense, control of the redox status of the cell and also roles during development. Previously, we reported the identification of five different MT homologues (PlMT4-8) from the Mediterranean sea urchin species P. lividus2. In order to shed some light on MT gene structure and evolution, we cloned seven P. lividus MT genes, comparing them to Echinodermata and Chordata genes3. Moreover, we performed a phylogenetic analysis of newly identified MTs from different classes of echinoderms and from the most ancient chordates, highlighting the relationships between them. Since MTs have multiple roles in the cells, we performed RT-qPCR and in situ hybridization experiments to understand better MT functions in sea urchin embryos. Results showed that the expression of MTs is regulated throughout development in a cell type-specific manner and in response to various metals3. The MT7 transcript is expressed in all tissues, especially in the stomach and in the intestine of the larva, but it is less metal-responsive. In contrast, MT8 is ectodermic and rises only at relatively high metal doses. MT5 and MT6 expression is highly stimulated by metals in the mesenchyme cells. Our results suggest that the P. lividus MT family originated by gene duplications and evolved developmental and environmental sub-functionalization. A preliminary in silico promoter analysis showed interesting similarities and differences among these genes.