Nucleotide sequence of a cDNA clone encoding an Arabidopsis thaliana thioredoxin h.

Thioredoxins are small proteins of approximately 12 kD that reduce disulfide bridges of target proteins by the reversible formation of a disulfide bridge between two neighboring Cys residues present in the active site (Try-Cys-Gly-Pro-Cys). Thioredoxins have been found to regulate a variety of biological reactions in prokaryotic and eukaryotic cells (Holmgren, 1985). In higher plants, two thioredoxin systems are known. The first system consists of two chloroplastic thioredoxins (m and f). They are reduced by an Fd-dependent thioredoxin reductase and regulate photosynthesis by activating NADP-malate dehydrogenase (type m) and enzymes of the photosynthetic carbon cycle including Fru-1,6-biphosphatase (type f) (Johnson et al., 1987; Muller, 1991). The second system is reduced by an NADPH-dependent thioredoxin reductase. This enzymic activity has been characterized in nongreen heterotrophic tissues, and, consequently, the proteins have been named thioredoxin h (Johnson et al., 1987; Florencio et al., 1988). One spinach thioredoxin h has been purified and partially sequenced (Marcus et al., 1991). We recently isolated one tobacco cDNA clone (Marty and Meyer, 1991) and one genomic clone (Brugidou et al., 1993) encoding two thioredoxin h proteins. In this paper, we report the nucleotide and deduced amino acid sequences of an Arabidopsis thaliana thioredoxin h. A cDNA library from immature siliques of A. thaliana cv Columbia was constructed by J. Giraudat (Institut des Sciences Végétales at Gif-Sur-Yvette) in X Zap 11. The library was screened with a radiolabeled 509-bp cDNA clone encoding the tobacco thioredoxin h l . Of 150,000 plaques, 12 positive plaques were obtained from the first round of screening. The positive plaques were purified by further screening at lower density. After in vivo plasmidization, the clones were sequenced. A11 are identical, containing the same 497-bp insert (Fig. 1). The identity values (Table I) indicate that thioredoxin h proteins are related and show more homology with the vertebrate cytoplasmic thioredoxins than with the chloroplastic ones.