The effects of 2000 R on protein and RNA synthesis of mouse liver mitochondria were determined. After 40-minute injection in vivo of14 C-leucine or3 H-uridine the uptake of both isotopes by the irradiated liver mitochondria was 80% and 36% higher respectively than control. Mitochondria from x-irradiated livers also incorporated 40% more14 C-leucine and 12% more${}^{3}{\rm H}\text{-}{\rm UTP}$ during 40 minutes incubation in vitro at 37°C than control. The distribution of nascent protein and RNA in submitochondrial fractions after in vitro incorporation of14 C-leucine or${}^{3}{\rm H}\text{-}{\rm UTP}$ indicated that the mitochondrial membrane fraction from x-irradiated livers incorporated 12.5% more${}^{3}{\rm H}\text{-}{\rm UTP}$ and 34% more14 C-leucine. The supernatant (excluding mito-ribosomal particles) incorporated 58.5% more${}^{3}{\rm H}\text{-}{\rm UTP}$ and 103% more14 C-leucine than control. The sedimentation profile of mito-ribosomes of control showed a higher proportion of particles in the 87S to 32S region and gave about 60% of the total14 C-leucine counts, whereas mito-ribosomal aggregates below 32S of the x-irradiated livers gave about 60% of the total14 C-leucine counts. Gel electrophoretic profiles of labeled mitochondrial RNA showed that about 45% higher 21S RNA and 50% higher 4S RNA were synthesized in x-irradiated liver mitochondria as compared to control. The higher levels of protein and RNA synthesis are explained by the increased transport of RNA and protein precursors due to change in mitochondrial membrane permeability induced by x-irradiation.

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