mRNAs encoding a novel thioredoxin were isolated from pollen RNA of Lolium perenne (LpTrx), Hordeum bulbosum (HbTrx), Phalaris coerulescens (PTrx) and Secale cereale (ScTrx). The cDNAs contain a single ORF of 393 bp encoding a protein of 131 amino acids. The predicted proteins showed highest homology to plant thioredoxins of the h class yet form a distinct subgroup that is characterized by a high level of sequence conservation (95.4-97.7% identity). GenBank searches revealed additional members of this subclass in tomato, soybean, rice and pine. LpTrx and PTrx were expressed as recombinant proteins in Escherichia coli and tested for thioredoxin activity. Both proteins displayed typical thioredoxin activity in the nonspecific insulin reduction assay, however, were not reduced by E. coli NADPH-dependant thioredoxin reductase.