Based on known amino acid sequences, probes have been generated by PCR and used for the subsequent isolation of cDNAs and genes coding for two thioredoxins (m and h) of Chlamydomonas reinhardtii. Thioredoxin m, a chloroplastic protein, is encoded as a preprotein of 140 amino acids (15,101 Da) containing a transit peptide of 34 amino acids with a very high content of Ala and Arg residues. The sequence for thioredoxin h codes for a 113 amino acid protein with a molecular mass of 11,817 Da and no signal sequence. The thioredoxin m gene contains a single intron and seems to be more archaic in structure than the thioredoxin h gene, which is split into 4 exons. The cDNA sequences encoding C. reinhardtii thioredoxins m and h have been integrated into the pET-3d expression vector, which permits efficient production of proteins in Escherichia coli cells. A high expression level of recombinant thioredoxins was obtained (up to 50 mg/l culture). This has allowed us to study the biochemical/biophysical properties of the two recombinant proteins. Interestingly, while the m-type thioredoxin was found to have characteristics very close to the ones of prokaryotic thioredoxins, the h-type thioredoxin was quite different with respect to its kinetic behaviour and, most strikingly, its heat denaturation properties.

NMR solution structures of a cytosolic plant thioredoxin h (112 amino acids, 11.7 kDa) from the green alga Chlamydomonas reinhardtii have been calculated on the basis of 1904 NMR distance restraints, which include 90 distances used to restrain 45 hydrogen bonds, and 44 4 dihedral restraints. The structure of C. reinhardtii thioredoxin h was solved in its oxidised form, and the ensemble of 23 converged structures superpose to the geometric average structure with an atomic rmsd of 0.080 nm ? 0.016 for the (N, Ca, C) backbone atoms of residues 4 -110. Comparisons with other thioredoxins, such as thioredoxin from the bacterium Escherichia coli, thioredoxin 2 from a cyanobacterium of the Anahaena genus, and human thioredoxin, showed that thioredoxin h models share more structural features with human thioredoxin than with other bacterial thioredoxins. Examination of the accessible surface around the redoxactive peptide sequence indicates that a potent thioredoxin-h -substrate interaction could be similar to the vertebrate thioredoxin-substrate interactions.