INTRON POSITION AS AN EVOLUTIONARY MARKER OF THIOREDOXINS AND THIOREDOXIN DOMAINS

In contrast to prokaryotes, which typically possess one thioredoxin ge ne per genome, three different thioredoxin types have been described i n higher plants, All are encoded by nuclear genes, but thioredoxins m and f are chloroplastic while thioredoxins h have no transit peptide a nd are probably cytoplasmic. We have cloned and sequenced Ambidopsis t haliana genomic fragments encoding the five previously described thior edoxins h, as well as a sixth gene encoding a new thioredoxin h. In sp ite of the high divergence of the sequences, five of them possess two introns at positions identical to the previously sequenced tobacco thi oredoxin h gene, while a single one has only the first intron. The rec ently published sequence of Chlamydomonas thioredoxin h shows three in trons, two at the same positions as in higher plants. This strongly su ggests a common origin for all cytoplasmic thioredoxins of plants and green algae. In addition, we have cloned and sequenced pea DNA genomic fragments encoding thioredoxins m and f. The thioredoxin m sequence s hows only one intron between the regions encoding the transit peptide and the mature protein, supporting the prokaryotic origin of this sequ ence and suggesting that its association with the transit peptide has been facilitated by exon shuffling. In contrast, the thioredoxin f seq uence shows two introns, one at the same position as an intron in vari ous plant and animal thioredoxins and the second at the same position as an intron in thioredoxin domains of disulfide isomerases. This stro ngly supports the hypothesis of a eukaryotic origin for chloroplastic thioredoxin f.