Regulation of oxidative stress response in Chlamydomonas reinhardtii
Beat B. Fischer1,2, Rik I.L. Eggen2, and Krishna K. Niyogi1
1) Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720
2) Department of Environmental Toxicology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
Reactive oxygen species (ROS) can function as signals to activate nuclear gene expression in photosynthetic organisms and stimulate an acclimation process increasing the tolerance of the organism to subsequent oxidative stress. A specific genetic response and acclimation process to the ROS singlet oxygen has been identified in the green alga Chlamydomonas reinhardtii. We now performed a mutant screen to isolate strains with either a modified response of the singlet oxygen-induced glutathione peroxidase gene GPXH or an altered tolerance to the singlet oxygen-producing photosensitizer rose bengal. Surprisingly, no mutants with deficient GPXH induction were found. However 20 strains with increased GPXH basal expression could be isolated of which most seem to have a stimulated singlet oxygen production under low light conditions probably due to some defects in protection mechanisms against photooxidative stress. Screening for mutants with altered tolerance to rose bengal resulted in the identification of sor1, an oxidative stress resistant strain which has constitutive high expression of the GPXH and several glutathione-S-transferase genes (GSTS1-3). The sor1 mutation was mapped to the gene of a 393-amino acid protein for which blast searches revealed only poor homology to other proteins but predicted a putative basic leucine zipper DNA binding domain indicating that it might function as transcription factor to activate gene expression during oxidative stress. Interestingly, SOR1 was also upregulated in the sor1 mutant, and a 8 bp palindromic sequence element identified in the promoter region of GPXH, GSTS1 and SOR1 was shown to be required for the overexpression of these genes. Thus, the SOR1 protein might be a novel transcription factor stimulating the expression of a specific set of genes as a response to cellular oxidative stress.
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