Characterization of an internal ribosomal entry segment in the 5 ' leader of murine leukemia virus env RNA

The 5' untranslated region, also called the leader, of oncoretroviruses and lentiviruses is long and is formed of several structured domains criticall y important in virus replication, The 5' leader of murine leukemia virus (M LV) RNA contains an internal ribosomal entry segment (IRES) which promotes synthesis of Gag and glyco-Gag polyprotein precursors. In the present study we investigated the translational features of the 5' leader of MLV subgeno mic RNA (env RNA) encoding the Env polyprotein precursor. When the env lead er was inserted between two genes, such as lacZ and the neomycin resistance cassette, in a dicistronic vector, it allowed IRES-dependent translation o f the 3' cistron in the rabbit reticulocyte lysate system and in murine cel ls. The drug rapamycin and the foot-and-mouth disease virus L protease, kno wn to inhibit cap-dependent translation, caused an enhancement of the trans lation driven by the env leader sequence, consistent with an IRES activity promoting Env expression. Analysis of several deletion mutants led us to lo calize the minimal env IRES between the splice junction and the env AUG sta rt codon.