Characterization of methionine sulfoxide reductases A and Bs from tobacco plant

One methionine sulfoxide reductase A (TMSRA) and two methionine sulfoxide reductase Bs (TMSRB1 and TMSRB2) were isolated from tobacco plants. TMSRA showed specificity for the reduction of Met-(S)-SO and both TMSRBs were specific for the reduction of Met-(R)-SO. TMSRA was the cytosolic form and both TMSRBs were plastid forms based on sequence comparison and expression tests. TMSRA and TMSRB2 could use either thioredoxin (TRX) or dithiothreitol (DTT) as reducing system, while TMSRB1 showed little activity with TRX but much more activity with DTT, which was similar to the mitochondrial MSRB2 from mammals. Ferredoxin (FD) is not the reducing system for Msrs, but might reflect the redox status in the cell and control the activity of Msrs indirectly.