Thiol Reductases in Deinococcus Bacteria and Roles in Stress Tolerance

Abstract

Deinococcus species possess remarkable tolerance to extreme environmental conditions that generate oxidative damage to macromolecules. Among enzymes fulfilling key functions in metabolism regulation and stress responses, thiol reductases (TRs) harbour catalytic cysteines modulating the redox status of Cys and Met in partner proteins. We present here a detailed description of Deinococcus TRs regarding gene occurrence, sequence features, and physiological functions that remain poorly characterised in this genus. Two NADPH-dependent thiol-based systems are present in Deinococcus. One involves thioredoxins, disulfide reductases providing electrons to protein partners involved notably in peroxide scavenging or in preserving protein redox status. The other is based on bacillithiol, a low-molecular-weight redox molecule, and bacilliredoxin, which together protect Cys residues against overoxidation. Deinococcus species possess various types of thiol peroxidases whose electron supply depends either on NADPH via thioredoxins or on NADH via lipoylated proteins. Recent data gained on deletion mutants confirmed the importance of TRs in Deinococcus tolerance to oxidative treatments, but additional investigations are needed to delineate the redox network in which they operate, and their precise physiological roles. The large palette of Deinococcus TR representatives very likely constitutes an asset for the maintenance of redox homeostasis in harsh stress conditions.

Keywords: Deinococcus; bacillithiol; cysteine; oxidative stress; peroxidase; protein redox status; reductase; thiol; thioredoxin.

Figure 1

The Trx system. (a) NADPH-dependent electron transfer via Trx. (b) Catalytic mechanism of substrate reduction by Trx. (c) Trx thiol reductase substrates involved in the maintenance of cell redox homeostasis. TrxR, thioredoxin reductase; Trx, thioredoxin; Prx, peroxiredoxin; Msr, methionine sulfoxide reductase; S_CH, catalytic cysteine thiol; S_RH, resolving cysteine thiol; MetO, methionine sulfoxide.

Figure 2

Multiple sequence alignments of TRX-like proteins from Deinococcus. (a) Alignment of Deinococcus DR_2085-type sequences. (b) Alignment of Deinococcus DR_2085 sequence with Homo sapiens (Hs) and Helicobacter pylori (Hp) homologous TRP14 Trxs. (c) Alignment of Deinococcus DR_0057 with Arabidopsis thaliana (At), Mus musculus (Mm), Saccharomyces cerevisiae (Sc), Haloplanus aerogenes (He) and Chlorobium ferrooxidans (Cf) homologous proteins. Trx active site sequences are highlighted in yellow. An asterisk (*) indicates positions which have a single, fully conserved residue; colon (:) and period (.) indicate conservation between groups of strongly and weakly similar properties, respectively. Abbreviations of Deinococcus species names are as specified in Table 1. Alignments were generated using UniProt ClustalO (https://www.uniprot.org/align/; accessed on 14 February 2022).

Figure 3

Electron sources for Deinococcus NADPH- and NADH-dependent thiol peroxidases (BCP, AhpE, AhpD, Ohr, OsmC) scavenging hydrogen and organic peroxides and peroxynitrites and participating in redox signalling. TrxR, thioredoxin reductase; Trx, thioredoxin; BCP, bacterioferritin comigratory protein; SucB, dihydrolipoamide succinyltransferase; PDHB, pyruvate dehydrogenase beta; Ahp, alkyl hydroperoxide reductase; Ohr, organic hydroperoxide resistance enzyme; OsmC, osmotically induced bacterial protein C. Dashed arrows indicate possible transfer of electrons.

Figure 4

The bacillithiol system. (a) Structure of bacillithiol (BSH), glycoside formed of L-cysteinyl-D-glucosamine and malic acid. (b) Electron transfer pathways in the BSH system. Bdr, NADPH–bacillithiol disulfide reductase. Brx, bacilliredoxin; S-Ox, thiol oxidised (bacillithiolated) form; S-Red, thiol reduced form.

Figure 5

Relationships and possible interplays between Deinococcus thiol reductase systems. Dashed arrows indicate possible transfer of electrons between Trx-, BSH-, and lipoylated-dependent systems. TrxR, thioredoxin reductase; Trx, thioredoxin; Bdr, NADPH–BSH disulfide reductase; BSH, bacillithiol; Brx, bacilliredoxin; Ahp, alkyl hydroperoxide reductase; Ohr, organic hydroperoxide resistance enzyme; OsmC, osmotically induced bacterial protein C.