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A glutathione transferase (GST) related to the theta (T) class of enzymes found in plants and animals has been cloned from the potato pathogen Phytophthora infestans. The cDNA encoded a 25kDa polypeptide termed PiGSTT1 which was expressed in E. coli as the native protein. The purified recombinant enzyme behaved as a dimer (PiGSTT1-1) and while being unable to catalyse the glutathione conjugation of 1-chloro-2,4-dintrobenzene, was highly active as a glutathione peroxidase with organic hydroperoxide substrates. In addition to reducing the synthetic substrate cumene hydroperoxide, PiGSTT1-1 was shown to be highly active toward 9(S)-hydroperoxy-(10E,12Z,15Z)-octadecatrienoic acid=9(S)-HPOT, which is formed in potato plants during infection by P. infestans as a precursor of the antifungal oxylipin colnelenic acid. An antiserum was raised to PiGSTT1-1 and used to demonstrate that the respective …