| Many metabolic reactions, such as photosynthesis, photorespiration, and respiration involve the formation of reactive oxygen species (ROS), particularly under extreme environment conditions. ROS can cause serious damage to macromolecules such as proteins, lipids, or nucleic acids, eventually leading to pathological process. But it also can act as signals modulating gene expression, enzyme activities or defense reactions. For this, the reactivity of oxygen has to be controlled to maintain the cellular metabolism in a functional state.Organisms living under aerobic conditions have developed an array of antioxidative system to protect the cells from oxidative damages. An emerging family of nonheme peroxidases, called peroxiredoxins (Prxs), has been described in all kingdoms. These enzymes catalyze the reduction of either hydrogen peroxide (H2O2) or various alkyl hydroperoxides to water and the corresponding alcohol in the presence of a hydrogen donor such as thioredoxin (Trx), glutaredoxin (Grx), cyclophilin A, ect. Important new information on Prxs indicates plant-specific functions of Prx, for example, in the context of photosynthesis, phloem metabolism, and environmental stress. Within the peroxiredoxins, four clusters of related proteins are distinguished: 1-Cys Prx, 2-Cys Prx, Type II Prx and Prx Q. Prx Q, as representative of the fourth subgroup, was identified initially in Sedum linear as a homologue of Escherichis coli bacterioferritin co-migratory protein (BCP). Although the molecular and biochemical characterization of Prxs genes in plants is advancing rapidly, important questions need to be addressed in the future, particularly, concerning physiological roles and regulation.By large-scale partial sequencing of randomly selected cDNA clones from a NaCl-treated cDNA library of S. salsa, we obtained the partial fragment of Prx Q (genebank accession No. AW982132). We obtained the fragment of 5' ends by using 5'-RACE method. Two gene's specific primers were designed, and SsPrx Q cDNA fragment was amplified with RT-PCR. After cloning and sequencing analysis, we found that it had high homology with other peroxiredoxin Q. SsPrx Q contains an open reading frame of 645 bp, with a5'-untranslated region of 100 bp, and a 3'-untranslated region of 258 bp. The predicated molecular mass of the protein is 23.6 kD. The N-terminus 64 amino acids constitute the chloroplast transit peptide. Southern analysis indicates that the SsPrx Q is a unique gene in S. salsa genome. Analysis of mRNA levels by Northern blotting indicated that the expression level of SsPrx Q in S. salsa leaves was significantly increased after treated with NaCl and low temperature. In addition, SsPrx Q gene expression was up-regulated by mannitol, H2O2, methyl viologen, and ABA. These results show that this gene is a stress response gene or have an important role in stress tolerance. In order to identify the function of the gene, first, we expressed and purified the recombinant SsPrx Q protein in E.coli. To further analyze the enzymatic activity of the expressed Prx Q, PreScission Protease was used to digest GST fusion protein. The purified SsPrx Q protein showed a band with a molecular mass of about 24KD. In vitro analysis indicated that the protein could protect DNA integrity against oxidative damage and also exhibited a thioredoxin-dependent peroxidase activity. Secondly, the SsPrx Q gene was integrated into the expression vector pROK II, then the vector was introduced into Arabidopsis thaliana by floral dipping method. Transformants were screened with the media containing Kanamycin. PCR and Southern analysis suggested that it had integrated into the Arabidopsis genome. Northern blotting showed that the transgenic plants had different expression levels. Finally, we found that the transgenic plants grew better than the wild type plants under the salt and low temperature treatment. These results indicated that SsPrx Q may play an important role in protecting plants from environmental stresses.
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