Expression and characterization of monocot rice cytosolic CuZnSOD protein in dicot Arabidopsis

Cytosolic CuZnSOD removes deleterious superoxides from plant cells. In order to understand its function better, we sought to express a monocot CuZnSOD gene in transgenic Arabidopsis. We constructed a transgene using the CaMV 35S promoter to express a rice cytosolic CuZnSOD gene in Arabidopsis and generated over 200 transformants. A 16 kD polypeptide, the same size as the native rice CuZnSOD polypeptide, was detected in the transgenic Arabidopsis. Interestingly, two forms of rice CuZnSOD, rSODI and rSODII, having the same dimeric size, were detected in the transgenic plants. rSODII protein was relatively abundant but had low specific activity. In contrast, rSODI protein was relatively rare and had high specific activity. Inter-conversion of rSODI and rSODII could be achieved by the addition and removal of copper ions into the purified recombinant SOD and to the leaf extract of transgenic plants. Our analysis indicates that rSODI most likely corresponds to native rice CuZnSOD that has incorporated the Cu and Zn ions required for full activity, whereas the less active rSODII form may not have properly incorporated the necessary copper ions.