A cDNA coding for a thioredoxin h has been isolated from a xylem/phloem poplar cDNA library by RACE‐PCR. The nucleotide sequence called popTrx‐h2 is homologous to other thioredoxins h isolated from plants but differs from the other thioredoxins h by presenting a 30 amino acid long N‐terminus extension. A variant of this cDNA lacking the N‐terminal extension was also generated by PCR. Both cDNAs have been introduced into an expression plasmid (pET‐3d) and the recombinant proteins have been expressed to a high level and purified from Escherichia coli cells. Protein sequencing showed that a part of the N‐terminal extension was cleaved in the E. coli cells, with the first 19 amino acids missing, suggesting the presence of a putative cleavage site in the N‐terminal extension of popTrx‐h2. Both recombinant proteins display unusual catalytic properties compared to other thioredoxins h characterized so far, i.e. a weak reduction by Arabidopsis thaliana NADPH‐dependent thioredoxin reductase, and a weak activation of the chloroplastic NADP‐malate dehydrogenase, a non‐physiological target enzyme. Northern blot experiments indicate that the transcripts of popTrx‐h2 are present in leaves and roots, albeit at a lower level compared to the earlier characterized popTrx‐h1.
