Thiol/selenol peroxidases are ubiquitous nonheme peroxidases. They are divided into two major subfamilies: peroxiredoxins (PRXs) and glutathione peroxidases (GPXs). PRXs are present in diverse subcellular compartments and divided into four types: 2-cys PRX, 1-cys PRX, PRX-Q , and type II PRX (PRXII). In mammals, most GPXs are selenoenzymes containing a highly reactive selenocysteine in their active site while yeast and land plants are devoid of selenoproteins but contain nonselenium GPXs. The presence of a chloroplastic 2-cys PRX, a nonselenium GPX, and two selenium-dependent GPXs has been reported in the unicellular green alga Chlamydomonas reinhardtii. The availability of the Chlamydomonas genome sequence offers the opportunity to complete our knowledge on thiol/selenol peroxidases in this organism. In this article, Chlamydomonas PRX and GPX families are presented and compared to their counterparts in Arabidopsis, human, yeast, and Synechocystis sp. A summary of the current knowledge on each family of peroxidases, especially in photosynthetic organisms, phylogenetic analyses, and investigations of the putative subcellular localization of each protein and its relative expression level, on the basis of EST data, are presented. We show that Chlamydomonas PRX and GPX families share some similarities with other photosynthetic organisms but also with human cells. The data are discussed in view of recent results suggesting that these enzymes are important scavengers of reactive oxygen species (ROS) and reactive nitrogen species (RNS) but also play a role in ROS signaling. L IFE in an oxygen-rich environment has to deal with the danger of oxidative stress. During normal cell metabolism, reactive oxygen species (ROS) and reactive nitrogen species (RNS) are constantly produced, essentially by respiratory and photosynthetic electron transfer chains. These highly reactive molecules can react with many cell components and damage DNA, proteins, and lipids. Thus, their concentration has to be strictly controlled. For this purpose, aerobic organisms are equipped with nonenzymatic (ascorbate, glutathione, tocopherol, and carotenoid) or enzymatic (catalase, ascorbate peroxidase, superoxide dismutase, glutathione peroxidase, and peroxiredoxin) antioxidant systems to remove ROS from the cells. To control their concentrations, ROS and RNS have to be sensed. It has been established in many organisms that ROS, and especially hydrogen peroxide, are signaling molecules that diffuse across membranes and induce specific signal transduction pathways. Furthermore, ROS can also be produced on purpose by cells in response to several stimuli to function as second messengers inside the cell. Finally, ROS and RNS can control enzyme activities by triggering several post-translational modifications such as disulfide bond formation, thiol oxidation to sulfenic/sulfinic/sulfonic acid, glutathionylation, nitrosylation, or carbonylation.Thiol/selenol peroxidases have emerged, during recent years, as important scavengers of ROS/RNS but they ...

When exposed to strong sunlight, photosynthetic organisms encounter photooxidative stress by the increased production of reactive oxygen species causing harmful damages to proteins and membranes. Consequently, a fast and specific induction of defense mechanisms is required to protect the organism from cell death. In Chlamydomonas reinhardtii, the glutathione peroxidase homologous gene GPXH/GPX5 was shown to be specifically upregulated by singlet oxygen formed during high light conditions presumably to prevent the accumulation of lipid hydroperoxides and membrane damage. We now showed that the GPXH protein is a thioredoxin-dependent peroxidase catalyzing the reduction of hydrogen peroxide and organic hydroperoxides.Furthermore, the GPXH gene seems to encode a dual-targeted protein, predicted to be localized both in the chloroplast and the cytoplasm, which is active with either plastidic TRXy or cytosolic TRXh1. Putative dual-targeting is achieved by alternative transcription and translation start sites expressed independently from either a TATA-box or an Initiator core promoter. Expression of both transcripts was upregulated by photooxidative stress even though with different strengths. The induction required the presence of the core promoter sequences and multiple upstream regulatory elements including a Sp1-like element and an earlier identified CRE/AP-1 homologous sequence. This element was further characterized by mutation analysis but could not be confirmed to be a consensus CRE or AP1 element. Instead, it rather seems to be another member of the large group of TGAC-transcription factor binding sites found to be involved in the response of different genes to oxidative stress.

Identification and characterization of IgE-inducing antigens are important for elucidating the mechanisms involved in IgE-mediated immune responses in allergic diseases and parasite infections. While many allergens have been characterized, little is known about parasite antigens inducing specific IgE following infection. In order to identify antigens from the nematode Nippostrongylus brasiliensis, we generated an IgEproducing B cell hybridoma from N. brasiliensis-infected C57BL/6 mice and constructed a cDNA phage display library from N. brasiliensis. We successfully cloned and expressed an N. brasiliensis antigen (Nb-Ag1) that showed specific binding to anti-N. brasiliensis IgE. Nb-Ag1 localized to the pharynx of adult N. brasiliensis, suggesting that Nb-Ag1 is a potential pharyngeal gland antigen. Nb-Ag1-specific IgE could be detected in the serum of N. brasiliensis-infected mice, but only for a short time and only following a challenge infection. In contrast, local administration of Nb-Ag1 during primary, secondary and tertiary infections induced Nb-Ag1-specific IgE-mediated active cutaneous anaphylaxis. Therefore, amongst the high amounts of polyclonal total IgE, low levels of parasitespecific IgE responses are induced during primary helminth infections. Here, we show that even such low levels of parasite-specific IgE are sufficient to prime mast cells in vivo and mediate degranulation.

The expression of the GPXH gene, coding for a phospholipid hydroperoxide glutathione peroxidase homologous protein, was shown to be strongly induced in Chlamydomonas reinhardtii exposed to photooxidative stress conditions like high light illumination. Here, we show that the response of the GPXH gene to high‐light conditions is induced by two independent signals and mechanisms: first, there is a fast but transient transcriptional induction by singlet oxygen formed after charge recombination and energy transfer from the overexcited photosystem II reaction center to molecular oxygen. Second, upon very strong or ongoing photooxidative stress, GPXH upregulation is further stimulated presumably by an mRNA stabilization mechanism as a result of lipid hydroperoxide formation. This could be confirmed by imitating the high‐light response of GPXH by exposing cultures of C. reinhardtii to a mixture of the singlet oxygen producing photosensitizer rose bengal and organic hydroperoxides. Furthermore, determination of mRNA decay rates allowed simulating the GPXH responses to various stress conditions with a mathematical model. A preventive effect of singlet oxygen formation on the resistance against photooxidative stress and lipid peroxidation was shown by acclimation experiments. Thus, the consecutive regulation of GPXH expression by an early transcriptional upregulation by a primary (singlet oxygen formation) and a time‐dependent stabilization of mRNA levels by a secondary effect (lipid peroxidation) of high light stress might be an efficient mechanism to control the stress response.