Characterization of the 5′ internal ribosome entry site of Plautia stali intestine virus

The RNA genome of Plautia stali intestine virus (PSIV; Cripavirus, Dicistroviridae) contains two open reading frames, the first of which is preceded by a 570 nt untranslated region (5' UTR). The 5' UTR was confirmed to be an internal ribosome entry site (IRES) using an insect cell lysate translation system: translation of a second cistron increased 14-fold in the presence of the 5' UTR and a cap analogue did not inhibit translation of the second cistron. Deletion analysis showed that 349 bases corresponding to nt 225-573 in the PSIV genome were necessary for internal initiation. The PSIV 5' IRES did not function in rabbit reticulocyte lysate or wheatgerm translation systems; however, the intergenic IRES for capsid translation of PSIV was functional in both systems, indicating that the 5' IRES and the intergenic IRES have distinct requirements for their activities. Chemical and enzymic analyses of the 5' IRES of PSIV indicate that its structure is distinct from that of Rhopalosiphum padi virus. Because 5' IRES elements in some dicistroviruses have been reported to be active in plant and mammalian cell-free translation systems, there appears to be variation among dicistroviruses in the mechanism of translation initiation mediated by 5' IRES elements.