Heterologous complementation of yeast reveals a new putative function for chloroplast m-type thioredoxin

In the chloroplast of higher plants, two types of thioredoxins (TRX), namel y TRX m which shows high similarity to prokaryotic thioredoxins and TRX f w hich is more closely related to eukaryotic thioredoxins, have been found an d biochemically characterized, but little is known about their physiologica l specificity with respect to their target(s). Here, we tested, in vivo, th e ability of organelle-specific TRX from Arabidopsis thaliana to compensate for TRX deficiency of a Saccharomyces cerevisiae mutant strain. Seven plan t organellar TRX (four of the m type, two of the f type and a newly discove red TRX x of prokaryotic type) were expressed in yeast in a putative mature form. None of these heterologous TRX were able to restore growth on sulpha te or methionine sulphoxide of the mutant cells. When we tested their abili ty to rescue the oxidant-hypersensitive phenotype of the TRX-deficient stra in, we found that TRX m and TRX x, but not TRX f, affected the tolerance to oxidative stress induced by either hydrogen peroxide or an alkyl hydropero xide. Athm1, Athm2, Athm4 and Athx induced hydrogen peroxide tolerance like the endogenous yeast thioredoxins. Unexpectedly, Athm3 had a hypersensitiz ing effect towards oxidative stress. The presence of functional heterologou s TRX was checked in the recombinant clones tested, supporting distinct abi lities for organelle-specific plant TRX to compensate for TRX deficiency in yeast. We propose a new function for the prokaryotic-type chloroplastic TR X as an antioxidant and provide in vivo evidence for different roles of chl oroplastic TRX isoforms.