Translation of capped virus mRNA in encephalomyocarditis virus-infected cells

The pattern of protein synthesis in HeLa cells simultaneously infected with encephalomyocarditis virus (EMCV) and Semliki Forest virus (SFV) has been analysed throughout the time course of the infection. The ratio of the picornavirus protein gamma versus the togavirus late protein C increased when the m.o.i. of EMCV was raised, and the ratio of C/gamma increased with higher multiplicities of SFV. Under some conditions, the co-infected cells simultaneously synthesized the picornavirus and togavirus proteins, and the cells exclusively translated the capped 26S mRNA from SFV at the end of the co-infection experiment. The influence of the time of addition of the second virus on the relative translation of the capped and uncapped mRNAs was also studied. When HeLa cells were co-infected with 5 p.f.u./cell of EMCV and 200 p.f.u./cell of SFV, only the synthesis of SFV proteins was apparent, whereas if SFV was added 1 to 3 h later during the course of EMCV infection the cells synthesized picornavirus and togavirus proteins. If the cells were superinfected with SFV 1 h after EMCV addition, host protein synthesis was drastically inhibited after 3 h of EMCV infection. By 5 h post-infection both kinds of virus proteins were synthesized and at 7 h post-infection the cells preferentially translated the capped 26S mRNA from SFV. If cells were first infected with SFV (10 p.f.u./cell) and co-infected or superinfected at 1 h with EMCV (50 p.f.u./cell), shut-off of host protein synthesis occurred 3 h after infection and the cells synthesized both kinds of virus proteins. However, 9 h after infection the cells synthesized SFV proteins only. When double-infected HeLa cells were placed in a hypotonic medium, they mainly synthesized the togavirus late proteins, whereas under hypertonic conditions, they translated the picornavirus RNA exclusively. These results suggest that the two kinds of mRNAs (SFV 26S mRNA and EMCV 35S mRNA) are present in the infected cells and that the relative translation of each of them depends on the external ionic conditions.