In vitro synthesis, importation and processing of Mn-superoxide dismutase (SOD-3) into maize mitochondria

In vitro translation of maize scutellar polysome-bound RNA and poly(A)+RNA produces a precursor for the mitochondrial superoxide dismutase (SOD-3) of maize, which can be immunoprecipitated with SOD-3 monospecific antibodies. The precursor SOD-3 (preSOD-3) has both a greater molecular weight and a more positive isoelectric point than the mature (purified) SOD-3. These differences were not observed for the cytosolic isozyme (SOD-4) which was similarly synthesized and isolated. PreSOD-3 specifically associates with maize mitochondria when incubated in vitro, whereas SOD-4 does not. The ionophore valinomycin (at concentrations of over 1 microM) inhibits the uptake of preSOD-3 into mitochondria. The integrity of the mitochondrial preparations was determined by assay of oxygen consumption, citrate synthase, and cytochrome-c oxidase activity.