Reovirus infection inhibits the incorporation of H-thymidine into cellular DNA. We have now investigated several aspects of this inhibition in L-929 cells early (8 h) after infection at high multiplicity (200 to 250 p.f.u./cell). Using equilibrium sedimentation analysis of DNA sequentially labelled with density and radioactive analogues of thymidine, we find a 52% reduction in the amount of DNA synthesized with no change in rate of replication fork movement in infected cells. Gel electrophoresis of histones labelled with H-lysine shows that infection inhibits their synthesis by 76% several hours before overall cellular protein synthesis is inhibited. There is also a reduction of nearly 50% in the size of the thymidine triphosphate pool, as measured by enzymic assay. The proportion of exogenous nucleotide in the pool is the same as in uninfected cells since there is no change in the buoyant density of DNA labelled during a short pulse with H-bromodeoxyuridine. The uptake of thymidine is reduced, but its phosphorylation to thymidine triphosphate is normal. The findings provide direct evidence that DNA synthesis is inhibited early in infection. This inhibition is accompanied by other derangements of thymidine and chromatin metabolism suggesting that there is an early and specific attack by reovirus on nuclear function in infected cells.


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