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 stres
