Translational control of early protein synthesis at the late stage of vaccinia virus infection

Abstract

The cytoplasmic RNA obtained at 1 hr (early) or 6 hr (late) after infection of Ehrlich ascites tumor (EAT) cells with vaccinia virus stimulated to similar extent the incorporation of [ 35S]methionine into proteins when translated in a micrococcal nuclease-treated lysate obtained also from EAT cells. Comparison on SDS-polyacrylamide gels of the in vitro translation products at nonsaturating RNA concentrations with those obtained in vivo by pulse labeling the infected cells showed that essentially all the early in vivo labeled proteins were made upon translation of the early RNA. Analysis of the cells pulse labeled in vivo at the late time demonstrated the presence of late proteins and of only residual amounts of some early proteins. The in vitro translation products of the late RNA, however, not only contained the late proteins but also appreciable amounts of several early proteins. This observation suggested that some early mRNA species were still present at the late time after infection but not translated in vivo, at least not to the same relative extent. In parallel, hybridization competition experiments revealed that the late cytoplasmic RNA contained almost twice as much early sequences as did the early cytoplasmic RNA. These results suggest that at the late stage of vaccinia virus infection the synthesis of the early proteins is subjected to translational control.