Cellular functions and transcriptional regulation of a third thioredoxin from Schizosaccharomyces pombe

Synergistic effect of thioredoxin and its reductase from Kluyveromyces marxianus on enhanced tolerance to multiple lignocellulose-derived inhibitors

BACKGROUND

Multiple lignocellulose-derived inhibitors represent great challenges for bioethanol production from lignocellulosic materials. These inhibitors that are related to the levels of intracellular reactive oxidative species (ROS) make oxidoreductases a potential target for an enhanced tolerance in yeasts.

RESULTS

In this study, the thioredoxin and its reductase from Kluyveromyces marxianus Y179 was identified, which was subsequently achieved over-expression in Saccharomyces cerevisiae 280. In spite of the negative effects by expression of thioredoxin gene (KmTRX), the thioredoxin reductase (KmTrxR) helped to enhance tolerance to multiple lignocellulose-derived inhibitors, such as formic acid and acetic acid. In particular, compared with each gene expression, the double over-expression of KmTRX2 and KmTrxR achieved a better ethanol fermentative profiles under a mixture of formic acid, acetic acid, and furfural (FAF) with a shorter lag period. At last, the mechanism that improves the tolerance depended on a normal level of intracellular ROS for cell survival under stress.

CONCLUSIONS

The synergistic effect of KmTrxR and KmTRX2 provided the potential possibility for ethanol production from lignocellulosic materials, and give a general insight into the possible toxicity mechanisms for further theoretical research.

Lack of a peroxiredoxin suppresses the lethality of cells devoid of electron donors by channelling electrons to oxidized ribonucleotide reductase

The thioredoxin and glutaredoxin pathways are responsible of recycling several enzymes which undergo intramolecular disulfide bond formation as part of their catalytic cycles such as the peroxide scavengers peroxiredoxins or the enzyme ribonucleotide reductase (RNR). RNR, the rate-limiting enzyme of deoxyribonucleotide synthesis, is an essential enzyme relying on these electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels, but little is known about the participation of electron donors in such regulation. Here, we show that cytosolic thioredoxins Trx1 and Trx3 are the primary electron donors for RNR in fission yeast. Unexpectedly, trx1 transcript and Trx1 protein levels are up-regulated in a G1-to-S phase-dependent manner, indicating that the supply of electron donors is also cell cycle-regulated. Indeed, genetic depletion of thioredoxins triggers a DNA replication checkpoint ruled by Rad3 and Cds1, with the final goal of up-regulating transcription of S phase genes and constitutive RNR synthesis. Regarding the thioredoxin and glutaredoxin cascades, one combination of gene deletions is synthetic lethal in fission yeast: cells lacking both thioredoxin reductase and cytosolic dithiol glutaredoxin. We have isolated a suppressor of this lethal phenotype: a mutation at the Tpx1-coding gene, leading to a frame shift and a loss-of-function of Tpx1, the main client of electron donors. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate such as the peroxiredoxin Tpx1 has been selected as a lethality suppressor to favor RNR function at the expense of the non-essential peroxide scavenging function, to allow DNA synthesis and cell growth.

The essential enzyme ribonucleotide reductase (RNR), the rate-limiting enzyme of deoxyribonucleotide synthesis, relies on the thioredoxin and glutaredoxin electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels. Here, we show that cytosolic thioredoxin Trx1 is the primary electron donor for RNR in fission yeast, and that trx1 transcript and protein levels are up-regulated at G1-to-S phase transition. Genetic depletion of thioredoxins triggers the DNA replication checkpoint up-regulating RNR synthesis. Furthermore, deletion of the genes coding for thioredoxin reductase and dithiol glutaredoxin is synthetic lethal, and we show that a loss-of-function mutation at the peroxiredoxin Tpx1-coding gene acts as a genetic suppressor. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate of redoxins, the peroxiredoxin Tpx1, has been selected as a lethality suppressor to favor channeling of electrons to the essential RNR.

Txl1 and Txc1 are co-factors of the 26S proteasome in fission yeast

The 26S proteasome is a large proteolytic particle present in the cytosol and nucleus of eukaryotic cells. Most intracellular proteins, including those affected by oxidative damage, are degraded by the proteasome. The human thioredoxin, Txnl1, is known to associate with the 26S proteasome and thereby equips proteasomes with redox capabilities. Here, we characterize the fission yeast orthologue of Txnl1, called Txl1. Txl1 associates with the 26S proteasome via its C-terminal domain. This domain is also found in the uncharacterized protein, Txc1, which was also found to interact with 26S proteasomes. A txl1 null mutant, but not a txc1 null, displayed a synthetic growth defect with cut8, encoding a protein that tethers the proteasome to the nuclear membrane. Txc1 is present throughout the cytoplasm and nucleus, whereas Txl1 co-localizes with 26S proteasomes in both wild-type cells and in cut8 mutants, indicating that Txl1 is tightly associated with 26S proteasomes, while Txc1 might be only transiently bound to the complex. Finally, we show that Txl1 is an active thioredoxin. Accordingly, Txl1 was able to reduce and mediate the degradation of an oxidized model proteasome substrate in vitro. Thus, Txl1 and Txc1 are proteasome co-factors connected with oxidative stress.

Thiol-independent action of mitochondrial thioredoxin to support the urea cycle of arginine biosynthesis in Schizosaccharomyces pombe

Thioredoxins usually perform a role as a thiol-disulfide oxidoreductase using their active-site cysteines. The fission yeast Schizosaccharomyces pombe contains two thioredoxins: Trx1 for general stress protection and Trx2 for mitochondrial functions. The Deltatrx2 mutant grows as well as the wild type on complex media containing glucose. However, on nonfermentable carbon source such as glycerol, the mutant did not grow, indicating a defect in mitochondrial function. The mutant also exhibited auxotrophy for arginine and cysteine on minimal medium. In order to find the reason for the unexpected arginine auxotrophy, we searched for multicopy suppressors and found that the arg3(+) gene encoding ornithine carbamoyltransferase (OCTase) in the urea cycle of the arginine biosynthetic pathway rescued the arginine auxotrophy. The levels of arg3(+) transcript, Arg3 protein, and OCTase activity were all decreased in Deltatrx2. Through immunocoprecipitation, we observed a direct interaction between Trx2 and Arg3 in cell extracts. The mutant forms of Trx2 lacking either one or both of the active site cysteines through substitution to serines also rescued the arginine auxotrophy and restored the decreased OCTase activity. They also rescued the growth defect of Deltatrx2 on glycerol medium. This contrasts with the thiol-dependent action of overproduced Trx2 in complementing glutathione reductase. Therefore, Trx2 serves multiple functions in mitochondria, protecting mitochondrial components against thiol-oxidative damage as a thiol-disulfide oxidoreductase, and supporting urea cycle and respiration in mitochondria in a manner independent of active site thiols.

The role and regulation of Trxl, a cytosolic thioredoxin in Schizosaccharomyces pombe

The genome of fission yeast Schizosaccharomyces pombe harbors two genes for thioredoxins, trx1(+) and trx2(+), which encode cytosolic and mitochondrial thioredoxins, respectively. The Deltatrx1 mutant was found sensitive to diverse external stressors such as various oxidants, heat, and salt, whereas Deltatrx2 mutant was not sensitive except to paraquat, a superoxide generator. Both Deltatrx1 and Deltatrx2 mutants were more resistant to diamide, a thiol-specific oxidant, than the wild type. The trx1(+) gene expression was induced by H(2)O(2) and menadione, being mediated through a stress-responsive transcription factor Papl. In Deltatrx1 cells, the basal expression of Pap1-regulated genes were elevated, suggesting a role for Trxl as a reducer for oxidized (activated) Papl. The Deltatrx1 mutant exhibited cysteine auxotrophy, which can be overcome by adding sulfite. This suggests that Trxl serves as a primary electron donor for 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase and thus is an essential protein for sulfur assimilation in S. pombe. These results suggest that, in contrast to Trx2 whose role is more confined to mitochondrial functions, Trxl plays a major role in protecting S. pombe against various stressful conditions and enables proper sulfur metabolism.