Structural and evolutionary aspects of thioredoxin reductases in photosynthetic organisms Jean-Pierre Jacquot 1 , Hans Eklund 2 , Nicolas Rouhier 1 and Peter Schu ¨ rmann 3 1 Interactions Arbres Microorganismes UMR 1136, IFR 110, Nancy University, BP 239, 54506 Vandoeuvre Cedex, France 2 Department of Molecular Biology, Swedish University of Agricultural Sciences, Biomedical Center, S-751 24 Uppsala, Sweden 3 Laboratoire de Biologie Mole ´ culaire et Cellulaire, Universite ´ de Neucha ˆ tel, Rue Emile-Argand 11, CP 158, CH-2009 Neucha ˆ tel, Switzerland Thioredoxins (Trxs) are small oxidoreductases that are involved in redox homeostasis and are found in large numbers in the subcellular compartments of eukaryotic plant cells, including the chloroplasts. Also present in chloroplasts are two forms of thioredoxin reductase (TR), which use either NADPH or ferredoxin as an elec- tron donor. In other compartments, two additional TR forms also use NADPH: one is distributed in all photo- synthetic organisms and is similar to prokaryotic enzymes, whereas the other is restricted to algae and is similar to mammalian selenoproteins. Here, we review current knowledge of the different forms of TRs across organisms and discuss the possible evolutionary fate of this class of enzymes, which provide an example of convergent functional evolution. Thioredoxin isoforms in plants: their subcellular localization and mode of reduction Thioredoxins (Trxs) are small oxidoreductases that are involved in the redox homeostasis of the cell and have a high catalytic activity for dithiol disulfide exchange reac- tions [1]. These reactions are essential for many cellular processes, including the regulation of Calvin cycle enzymes by light [1,2]. Unlike non-photosynthetic organisms, eukaryotic plants contain a large number of Trx isoforms located in several subcellular compartments, including the cytosol, mitochondria, chloroplasts (plastids) and nuclei [3–6]. Recent phylogenetic analyses have classified plant Trxs into 15 categories, and 49 and 41 genes of Trx and Trx- like sequences have been found in Populus trichocarpa and Arabidopsis thaliana, respectively [7]. Chloroplasts and plastids contain the f-, m-, x-, y- and chloroplast drought- induced stress protein (CDSP)32-type Trxs; h-type Trxs are present in cytosol and mitochondria; and o-type Trxs are present only in mitochondria [3,4]. The f-, m- and h-type Trxs, as well as several of their target proteins, have been characterized structurally up to the 3D level [8– 10]. By contrast, other plant Trxs, such as Clot, HCF164 (high chlorophyll fluorescence 164), nucleoredoxin, Trx- like type and lilium-type Trxs, are as yet uncharacterized [7]. The reasons for the expansion of Trx and Trx-like sequences in plants compared with other living organisms are still uncertain but could be related to the increased generation of reactive oxygen species (ROS) linked to the photosynthetic electron transfer chain and also to genome duplications in terrestrial plants [4]. Because plants con- tain many Trxs in different subcellular compartments, several reduction systems are also present in those com- partments. Chloroplasts contain two thioredoxin reductases (TRs). The first, ferredoxin:thioredoxin reductase (FTR), is a heterodimer with an iron–sulfur (Fe–S) center and a dis- ulfide that is reduced by ferredoxin (Fdx) [10]. The second, NADPH-dependent thioredoxin reductase (NTR) of chlor- oplasts (NTRC), is reduced by NADPH and is a fusion between a TR module and a Trx module [11,12]. In cytosol and mitochondria of higher plants, the reducing system is NADPH dependent, and, like NTRC, prokaryotic-type NTR contains a flavin adenine dinucleotide (FAD) cofactor and a disulfide at the active site [13]. In addition, diatoms, haptophytes and most green algae have an elongated form of NTR, similar to the mammalian NTRs, which contains a FAD and a selenosulfide active site [14,15]. Trxs constitute a prototype of small oxidoreductases with a specific fold and a conserved active site (WCGPC, with some variations) [1]. They have given rise to a variety of related macromolecules with similar architecture but with different active sites, namely glutaredoxins (Grxs, typically with a YCPYC active site, with many variations) and protein disulfide isomerases (PDIs, classically with a WCGHC active site) [16,17]. Grxs and PDIs have less electronegative redox potentials than do Trxs and are involved in different reactions, such as deglutathionylation and protein folding, respectively [18]. Interestingly, Grxs are generally reduced via glutathione (GSH), the reduction of which is maintained by glutathione reductase (GR) and NADPH. Structural analyses have revealed that NTRs and GRs have high homology at the structural level, although they are not closely related at the primary sequence level [19]. Here, we review current knowledge about the different forms of TRs across photosynthetic organisms. In addition, by analyzing the arrangement of genes encoding TR and redoxins (Trxs, Grxs, Fdxs and peroxiredoxins [Prxs]) in prokaryote genomes and the existence of genes encoding fusion proteins between TR and redoxins both in Review Corresponding author: Jacquot, J.-P. (jean-pierre.jacquot@scbiol.uhp-nancy.fr). 336 1360-1385/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2009.03.005 Available online 14 May 2009