Distinct modes of poliovirus polyprotein initiation in vitro.

RNAs of poliovirus type 1 and type 3 were translated in extracts from Krebs-2 cells after annealing with oligodeoxyribonucleotides complementary to different sites in the 5'-untranslated region (5'-UTR). Due to a high level of endogenous RNase H activity in the extracts, such RNAs appeared to be efficiently 5'-truncated prior to translation. The observed levels of initiation on differently truncated templates suggested that a region in the middle of the poliovirus 5'-UTR is essential for the cap-independent initiation of viral polyprotein synthesis. The data reported here, in conjunction with the results from other laboratories, permitted to relate the essential cis-acting control elements to the 5'-UTR secondary structure domains defined previously (E.V. Pilipenko et al., Virology 168, 201-209). However, the removal of these domains activated another mode of polyprotein initiation, which appeared to require another set of translation initiation factors.