Evidence for five divergent thioredoxin h sequences in Arabidopsis thaliana

Five different clones encoding thioredoxin homologues were isolated from Arabidopsis thaliana cDNA libraries. On the basis of the sequences they encode divergent proteins, but all belong to the cytoplasmic thioredoxins h previously described in higher plants. The five proteins obtained by overexpressing the coding sequences in Escherichia coli present typical thioredoxin activities (NADP(+)-malate dehydrogenase activation and reduction by Arabidopsis thioredoxin reductase) despite the presence of a variant active site, Trp-Cys-Pro-Pro-Cys, in three proteins in place of the canonical Trp-Cys-Gly-Pro-Cys sequence described for thioredoxins in prokaryotes and eukaryotes. Southern blots show that each cDNA is encoded by a single gene but suggest the presence of additional related sequences in the Arabidopsis genome. This very complex diversity of thioredoxins h is probably common to all higher plants, since the Arabidopsis sequences appear to have diverged very early, at the beginning of plant speciation. This diversity allows the transduction of a redox signal into multiple pathways.