The 3' Untranslated Region of a Plant Viral RNA Directs Efficient Cap-Independent Translation in Plant and Mammalian Systems.

Many plant viral RNA genomes lack a 5' cap, and instead are translated via a cap-independent translation element (CITE) in the 3' untranslated region (UTR). The panicum mosaic virus-like CITE (PTE), found in many plant viral RNAs, binds and requires the cap-binding translation initiation factor eIF4E to facilitate translation. eIF4E is structurally conserved between plants and animals, so we tested cap-independent translation efficiency of PTEs of nine plant viruses in plant and mammalian systems. The PTE from thin paspalum asymptomatic virus (TPAV) facilitated efficient cap-independent translation in wheat germ extract, rabbit reticulocyte lysate, HeLa cell lysate, and in oat and mammalian (BHK) cells. Human eIF4E bound the TPAV PTE but not a PTE that did not stimulate cap-independent translation in mammalian extracts or cells. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) footprinting revealed that both human and wheat eIF4E protected the conserved guanosine (G)-rich domain in the TPAV PTE pseudoknot. The central G plays a key role, as it was found to be required for translation and protection from SHAPE modification by eIF4E. These results provide insight on how plant viruses gain access to the host's translational machinery, an essential step in infection, and raise the possibility that similar PTE-like mechanisms may exist in mRNAs of mammals or their viruses.