The thioredoxin system, formed by thioredoxin reductase (TrxR) and its substrate thioredoxin (Trx) catalyzes thiol-disulphide reactions to maintain the cellular redox homeostasis. Two main thioredoxin systems have been described in mammals, one localized in the cytoplasm (Trx-1 and TrxR-1) and the other one found in mitochondria (Trx-2 and TrxR-2). Different functions have been attributed to both thioredoxin systems including protection against oxidative stress, regulation of cellular differentiation and apoptosis. In C. elegans the mitochondrial thioredoxin system is composed of the ZK637.10/trxr-2 and B0024.9/trx-2 genes. The
trx-2 and
trxr-2 genes encode mitochondrial proteins as transgenic strains expressing translational GFP fusions display a general punctuated fluorescence in most worm tissues. Co-localization with the mitochondrial marker Mitotracker confirmed the mitochondrial localization of TRX-2 and TRXR-2. As
trxr-2 is the third gene of an operon, transgenic strains expressing translational GFP fusions under the control of the putative internal promoter also resulted in fluorescence with an expression pattern different from that of the first gene of the operon. By RT-PCR and western blot, we showed that two different
trxr-2 alleles (
tm2047) and (
ok2267) are null mutants as they do not express
trxr-2 mRNA or protein. In contrast, the
trx-2 (
tm2720) allele expresses an abnormal mRNA, which encodes an anomalous TRX-2 with an aberrant C-terminus. Double mutants
trxr-2(
tm2047) III;
trx-2(
tm2720) V and
trxr-2(
ok2267) III;
trx-2(
tm2720) V are viable, suggesting that another redox system acts redundantly to the mitochondrial thioredoxin system. Double mutants of the mitochondrial genes
trx-2 and
trxr-2 with the cytoplasmic thioredoxin reductase
trxr-1 are also viable. We studied whether embryonic development or fertility was compromised in
trx-2 and
trxr-2 mutants, as seen in some other organisms, and observed only a mild decrease of fertility, but no embryonic lethality in the
trxr-2 (
ok2267) mutant. As thioredoxin systems are among the most important defence mechanisms against oxidative stress, we evaluated the role of
trx-2 and
trxr-2 in coping with paraquat-induced oxidative stress. Single and double mutants of the mitochondrial thioredoxin system have an increased resistance when compared to wild type, suggesting that the lack of the mitochondrial thioredoxin system might induce other antioxidant defences.