Chloroplastic Glucose 6-Phosphate Dehydrogenase from Chlorella vulgaris Alleviates Freezing and Menadione-Induced Oxidative Stresses in Saccharomyces cerevisiae

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Enhanced glucose 6-phosphate dehydrogenase (E.C.1.1.1.49, G6PDH) activity has been identified as a hardening-induced intracellular change of Chlorella vulgaris, which acquires freezing tolerance during hardening. In the present study, a full-length cDNA clone corresponding to a gene encoding a chloroplastic isoform of G6PDH, designated CvchG6PDH, was isolated from C. vulgaris C-27. By comparing the deduced amino acid sequence of CvchG6PDH with the N-terminal amino acid sequence of mature G6PDH_2 protein isolated previously, a DNA region encoding mature CvchG6PDH was determined and designated mCvchG6PDH. The deduced amino acid sequence of CvchG6PDH showed higher homology to those of plant plastidic G6PDH genes than those of cytosolic ones. A recombinant mCVchG6PDH protein expressed in Escherichia coli showed similar enzymatic properties to previously-isolated Chlorella G6PDH_2, suggesting that the gene encoded plastidic G6PDH_2 protein. Expression of CvchG6PDH was induced transcriptionally throughout 24-h hardening, while the translation was induced up to 9-h hardening and then decreased, and the change did not reflect the enhanced G6PDH activity during hardening. Furthermore, the mCvchG6PDH alleviated both freezing and menadione-induced oxidative stresses in yeast. We showed the contribution of CvchG6PDH in menadione stress tolerance as one of its functions in the acquisition of freezing tolerance of Chlorella.
2010-02-26