From heavy metal‐binders to biosensors: Ciliate metallothioneins discussed

Abstract

Metallothioneins (MTs) are ubiquitous proteins with the capacity to bind heavy metal ions (mainly Cd, Zn or Cu), and they have been found in animals, plants, eukaryotic and prokaryotic micro-organisms. We have carried out a comparative analysis of ciliate MTs (Tetrahymena species) to well-known MTs from other organisms, discussing their exclusive features, such as the presence of aromatic amino acid residues and almost exclusive cysteine clusters (CCC) present in cadmium-binding metallothioneins (CdMTs), higher heavy metal-MT stoichiometry values, and a strictly conserved modular-submodular structure. Based on this last feature and an extensive gene duplication, we propose a possible model for the evolutionary history of T. thermophila MTs. We also suggest possible functions for these MTs from consideration of their differential gene expressions and discuss the potential use of these proteins and/or their gene promoters for designing molecular or whole-cell biosensors for a fast detection of heavy metals in diverse polluted ecosystems.

Isolation and characterization of a novel copper‐inducible metallothionein gene of a ciliate, Tetrahymena tropicalis lahorensis

“…11), after addition of the two new CuMT isoforms of T. t. lahorensis, corroborates the previous analysis [Diaz et al, 2007;Amaro et al, 2008;Gutierrez et al, 2009], which divides the Tetrahymena MT gene family 7 [Binz and Kagi, 1999; http://www.expasy.ch/cgi-bin/lists?metallo.txt] into two subfamilies-7a for CdMTs and 7b for CuMTs.…”

“…Contrary to the views of Gutierrez et al [2009] we believe that Cd MTs are more ancient proteins compared with CuMTs, and of the two isoforms TtlCuMT1 evolved earlier than the other isoform TtlCuMT2.…”

“…Ciliates use particular genetic code, the two codons TAA and TAG, which are universal stop codons in nuclear genes, code Gln, and TGA is the only stop codon used by these eukaryotic microorganisms [review by Gutierrez et al, 2009]. Both the isoforms of CuMT of T.t.…”

Assessing the resistance and bioremediation ability of selected bacterial and protozoan species to heavy metals in metal-rich industrial wastewater

“…Among these mechanisms, heavy metal efflux systems have been well-studied [12]. The efflux-mediated mechanism is basically a plasmid-encoded mechanism involving many operons such as czcD, chrB, nccA and so on, in which toxic ions enter the cell via active transport (an ATPase pump) or diffusion (a chemiosmotic ion or proton pump) [12,13]. However, this metal resistance ability is a direct response to the metal species concerned, and consequently, a particular organism may directly and/or indirectly rely on several survival strategies [11].…”

Functional GFP-metallothionein fusion protein from Tetrahymena thermophila: a potential whole-cell biosensor for monitoring heavy metal pollution and a cell model to study metallothionein overproduction effects

“…The ciliate T. thermophila has five MT isoforms; three CdMTs ( MTT1, MTT3 and MTT5 ) and two CuMTs ( MTT2 and MTT4 ) (Díaz et al, 2007; Gutierrez et al, 2009; 2011). The CdMT genes are expressed at very different levels in response to metal stress (Díaz et al, 2007).…”