Isolation and characterization of an extended thioredoxin h from poplar

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.