NADPH-dependent thioredoxin reductases (NTR) are homodimeric enzymes that reduce thioredoxins. Two genes encoding NADPH-dependent thioredoxin reductases (AtNTRA and AtNTRB) were found in the genome of Arabidopsis thaliana. These originated from a recent duplication event and the encoded proteins are highly homologous. Previously, AtNTRA was shown to encode a dual targeted cytosol and mitochondrial protein. Here, we show that the AtNTRB gene encodes two mRNAs, presumably by initiating transcription at two different sites. The longer mRNA encodes a precursor polypeptide that is actively imported into mitochondria by a cleavage-associated mechanism, while the shorter mRNA encodes a cytosolic isoform. Isolation of Arabidopsis mutants with knocked-out AtNTRA or AtNTRB genes allowed us to prove that both genes encode cytosolic and mitochondrial isoforms. Interestingly, AtNTRB appeared to express the major mitochondrial NTR, while AtNTRA expresses as the major cytosolic isoform, suggesting that these two recently duplicated genes are evolving towards a specific function. (C) 2004 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.