Studies on protein and nucleic acid metabolism in virus-infected mammalian cells. 4. The localization of metabolic changes within subcellular fractions of Krebs II mouse-ascites-tumour cells infected with encephalomyocarditis virus

Abstract

The effect of infection with encephalomyocarditis virus on the rate of incorporation of 14C-labelled precursors into the protein and RNA of the subcellular components of Krebs II ascites-tumor cells has been investigated. The nucleus, which was the major site of RNA synthesis in the cell, contained negligible amounts of virus. Infection caused a marked progressive inhibition of orotic acid incorporation into nuclear RNA, and also some net loss of RNA from the nucleus. It is suggested that this disruption of nuclear RNA metabolism is related to the cell-killing properties of the virus. Most of the virus sedimented with the mitochondrial fraction. The amount of mitochondrial RNA increased progressively during infection by an amount approximately equivalent to that lost from the nucleus. Incorporation of orotic acid into mitochondrial RNA was slightly inhibited for the first 3 hours after infection; thereafter it was enhanced, reaching 320% of the control value at 6 hours. Less virus appeared in the microsomal fraction. The pattern of incorporation into microsomal RNA was similar to that of the mitochondria, but less pronounced. In both fractions, the increase in incorporation rate was apparently related to the amount of virus present, but estimates showed that only 5-8% of this newly synthesized RNA could be ascribed to viral RNA formation. In all cytoplasmic fractions, infection caused an initial slight stimulation of valine incorporation into protein, which was most marked in the cell-sap fraction. This was followed by a period of moderate inhibition, until appreciable amounts of virus had accumulated intracellularly, when incorporation- into mitochondrial protein was again elevated.