Superinfection of equine herpesvirus type 1 (EHV-1)-infected rabbit kidney cells with heat-inactivated frog virus 3 (ΔFV3) differentially blocked EHV-1 protein synthesis. The extent of inhibition varied with the specific EHV-1 message, but in general late protein synthesis was inhibited more than early and immediate early translation. Since FV3 has been shown to block heterologous RNA and protein synthesis, it was necessary to determine whether the observed reduction in herpesvirus protein synthesis was primarily due to a block in translation or to an earlier inhibition of EHV-1 mRNA synthesis. To distinguish between these alternatives, replicate cultures of EHV-1 infected cells were either superinfected with ΔFV3 or treated with 10 μg/ml actinomycin D at 6 hr after infection, and EHV-1 protein synthesis monitored 3 hr later. We found that addition of actinomycin D to EHV-I infected cultures had only a slight effect on EHV-1 translation, whereas superinfection with ΔFV3 markedly reduced EHV-1 protein synthesis. This result suggested that the observed decline in EHV-1 protein synthesis was not due to the inhibition of herpesvirus mRNA synthesis. In addition, we showed that RNA extracted from ΔFV3-superinfected cells directed the synthesis of full-size EHV-1 proteins in otro indicating that shut-off was not caused by the degradation of EHV-1 mRNAs. Taken together these results show that ΔFV3 selectively inhibited EHV-1 protein synthesis and are consistent with earlier observations which suggest that translational shut-off occurs at initiation.
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