A plant manganese superoxide dismutase is efficiently imported and correctly processed by yeast mitochondria

In the plant Nicotiana plumbaginifolia, manganese superoxide dismutase (MnSOD) is synthesized in the cytoplasm as a preprotein and is subsequently translocated to the mitochondrial matrix with corresponding cleavage of an NH2-terminal leader sequence. To determine whether the plant enzyme could replace the endogenous SOD activities of Escherichia coli and yeast, constructions have been made in appropriate vectors for expression of the preprotein and the mature MnSOD. These were introduced into SOD-deficient strains for complementation studies. In E. coli, both forms of the protein were shown to be active and able to complement SOD deficiency to different degrees. Expression of the preprotein in a yeast strain lacking a mitochondrial MnSOD resulted in a restoration of wild-type growth, only possible if the plant protein was being targeted to the mitochondria. Subsequent studies revealed that the protein was processed and that the leader sequence was cleaved at the identical position as recognized by the mitochondrial peptidase of plants. The components mediating mitochondrial import thus appear to be highly conserved between plants and yeast.