REDOX CONTROL OF RNA-SYNTHESIS IN POTATO MITOCHONDRIA

This study shows that the incorporation of radiolabelled UTP into RNA in Percoll-gadient-purified potato mitochondria is regulated by the re dox state of the mitochondrial electron transport chain. An early indi cation that there might be a redox effect on RNA synthesis was a decre ase in UTP incorporation in incubates containing an oxidisable substra te, such as succinate or malate. Subsequent use of a variety of electr on transport inhibitors acting at different points in the electron tra nsport chain established that the redox state of the Rieske iron-sulph ur protein was the major determinant of UTP incorporation. Inhibitors acting on the substrate side of the Rieske iron-sulphur protein, and c ausing oxidation of components on the oxygen side of their site of act ion, increased UTP incorporation into RNA. These included antimycin A, myxothiazole, and undecylhydroxydioxobenzothiazole at 500 nM. Inhibit ors acting on the oxygen side of the Rieske iron-sulphur protein, and causing a reduction of components on the substrate side of the block. decreased UTP incorporation. These inhibitors were undecylhydroxpdioxo benzothiazole at 25 nM and KCN. When phenazine methosulphate was prese nt as an auto-oxidisable electron sink the effect of KCN was diminishe d. The conclusion from the inhibitor experiments that the redox state of the Rieske iron-suphur protein was important was supported when RNA synthesis was measured at a range of redox potentials. This gave a me asured redox potential for the control of UTP incorporation into RNA o f +270 mV and the slope of the curve indicated an n = 1 carrier This v alue is close to the reported value of the Rieske iron-suphur protein. UTP incorporation was decreased by some 50% in the presence of low co ncentrations of okadaic acid (5 nM), an inhibitor of protein phosphata ses PP1 and PP2A, and alpha-naphthyl acid phosphate, a broad-spectrum phosphatase inhibitor, indicating that the redox effect on RNA synthes is may be mediated via protein phosphorylation. We did not, however, d etect an expected increase in RNA synthesis when protein kinase inhibi tors were used, so the involvement of protein phosphorylation in the r edox regulation of RNA synthesis is as yet uncertain.