Molecular cloning and characterization of two isoforms of cytoplasmic/mitochondrial type NADPH-dependent thioredoxin reductase from rice (Oryza sativa L. ssp. indica)

An NADP/thioredoxin system, consisting of NADPH, NADPH-dependent thioredoxin reductase (NTR) and thioredoxin (Trx) plays a post-translational regulatory role by reducing disulfide bonds in target proteins involved in various cellular processes. Plants have a complex NTR/Trx system comprising several Trxs and NTR isoforms. Three genes encoding NTR were found in the genome of rice. OsNTRC is chloroplastic type NTR whereas OsNTRA and OsNTRB are cytoplasmic/mitochondrial type NTR. The presence of two isoforms of cytoplasmic/mitochondrial type NTR with high amino acid identity raises the question of their functional specificity. In the present study, we describe isolation and cloning of two cDNAs encoding OsNTRA and OsNTRB. Both isoforms were heterologously expressed in Escherichia coli and purified using affinity chromatography, enabling comparison of their NADPHdependent reduction activities. OsNTRB and OsNTRA exhibited almost similar in vitro activities. OsNTRB was highly expressed in roots and shoots of one, two and three week-old rice seedlings, whereas the expression of OsNTRA in shoots was much higher than roots. Differential expression of OsNTRA in different tissues suggests that this isoform may have different functions in different tissues.