The cytosolic thioredoxin peroxidase II (cTPxII/ Tsa2p) from Saccharomyces cerevisiae shares 86% identity with the relatively well characterized cytosolic thioredoxin peroxidase I (cTPxI/Tsa1p). In contrast to cTPxI protein, cTPxII is not abundant and is highly inducible by peroxides. Here, we describe a unique phenotype for ⌬cTPxII strain; these cells were highly sensitive to tert-butylhydroperoxide (TBHP) but presented resistance to H 2 O 2 in fermentative and respiratory conditions. In

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... st, ⌬cTPxI strain was very sensitive to both TBHP and H 2 O 2 , whatever the carbon source present in the media. These differences in the response of mutant cells to the different kinds of peroxide insult could not be attributed to enzymatic properties of cTPxI and cTPxII since the recombinant proteins showed similar in vitro efficiencies (K cat /K m ) in the removals of both kinds of peroxide. This specific sensitivity of ⌬cTPxII cells to TBHP could not be related to the expression pattern of TSA2 (cytosolic thioredoxin peroxidase II gene) either, since this gene is highly inducible by both H 2 O 2 and TBHP when cells were grown in different conditions. Finally, peroxide-removing assays were performed and showed that catalase activity increased significantly only in ⌬cTPxII cells, which appear to be related with the resistance of this strain to H 2 O 2 . Taken together, present data indicate that cTPxII and cTPxI are key components of the yeast defense system against organic peroxide insult. In regard to the stress induced by H 2 O 2 , catalases (peroxisomal and/or cytosolic) and cTPxII seemed to cooperate with cTPxI in the defense of yeast against this oxidant.