Ciliate metallothioneins: unique microbial eukaryotic heavy-metal-binder molecules

This article represents an updated review of ciliate metallothioneins (Tetrahymena species) including a comparative analysis with regard to well-known metallothioneins (MTs) from other organisms and discussion of their exclusive features. It opens with an introduction to ciliates, summarizing the main characteristics of these eukaryotic microorganisms and their use as cellular models to study metallothioneins and metal-eukaryotic cell interactions. It has been experimentally proved that at least three different metal resistance mechanisms exist in ciliates, of which bioaccumulation is the most studied. Structural comparative analysis reveals that Tetrahymena MTs have unique characteristics, such as longer length, a considerably higher cysteine content, different metal-MT stoichiometry values, the presence of new cysteine clusters, and a strictly conserved modular-submodular structure. Gene expression analysis reveals a multistress and differential response to diverse metals and other environmental stressors, which corroborates the classification of these MTs. An in silico analysis of the promoter sequences of some MT genes reveals the presence of conserved motifs that are probably involved in gene expression regulation. We also discuss the great advantages of the first ciliate whole-cell biosensors based on MT promoters from Tetrahymena thermophila to detect heavy metal ions in environmental samples.