The disruption of the two thioredoxin genes in Saccharomyces cerevisiae leads to a complex phenotype, including the inability to use methionine sulfoxide as sulfur source, modified cell cycle parameters, reduced H(2)O(2) tolerance, and inability to use sulfate as sulfur source. Expression of one of the multiple Arabidopsis thaliana thioredoxins h in this mutant complements only some aspects of the phenotype, depending on the expressed thioredoxin: AtTRX2 or AtTRX3 induce methionine sulfoxide assimilation and restore a normal cell cycle. In addition AtTRX2 also confers growth on sulfate but no H(2)O(2) tolerance. In contrast, AtTRX3 does not confer growth on sulfate but induces H(2)O(2) tolerance. We have constructed hybrid proteins between these two thioredoxins and show that all information necessary for sulfate assimilation is present in the C-terminal part of AtTRX2, whereas some information needed for H(2)O(2) tolerance is located in the N-terminal part of AtTRX3. In addition, mutation of the atypical redox active site WCPPC to the classical site WCGPC restores some growth on sulfate. All these data suggest that the multiple Arabidopsis thioredoxins h originate from a totipotent ancestor with all the determinants necessary for interaction with the different thioredoxin target proteins. After duplications each member evolved by losing or masking some of the determinants.