| Viruses are obligate parasites that require host factors at every stage of infection, including viral translation, where they rely on host ribosomes and translation factors. While most cellular mRNAs use a 5' cap-dependent mechanism to undergo translation, many viruses have evolved diverse mechanisms to circumvent the traditional host requirements, compete against their hosts, and favor their own translation. Although all members of the largest family of plant-infecting viruses, the Potyviridae, use their 5' untranslated regions (5' UTRs) to conduct translation cap-independently, their mechanisms are diverse, vary depending on the virus and/or host, and are poorly understood. To help fill this knowledge gap, this research focused on the translation mechanism of a recently-emerged wheat-infecting virus, Triticum mosaic virus (TriMV), which initiates its translation using a novel, cap-independent translation element located within its exceptionally long (739-nt) 5' UTR. The TriMV 5' UTR drives cap-independent and 5' end-independent translation internally on the viral RNA, both in wheat germ extract and oat protoplasts. 5' UTR-driven translation is uninhibited when a strong hairpin is placed at the 5' end of a monocistronic reporter RNA, and thus the 5' UTR displays internal translation initiation capabilities. Although the 5' UTR sequence uniquely contains 13 AUG start codons, translation initiation primarily occurs at the 13th AUG. The TriMV 5' UTR translates independently of the cap-binding protein eIF4E and its isoform eIFiso4E, but requires the scaffold protein eIF4G/eIFiso4G. While the TriMV 5' UTR RNA binds to eIF4G with higher affinity than eIFiso4G, either isoform may be used for viral translation. We also provide evidence that the helicase eIF4A, which is a component of the cap-binding complex, is required for TriMV translation. To investigate the RNA structural requirements for 5' UTR function, we identified a stem-loop structure at nucleotide positions 469-490 that is required to bind the translation factors, drive internal translation, and compete against a capped RNA. When this stem loop is disrupted, all of these activities are lost. Our results reveal a novel viral translation element that may provide new insights into virus-host interactions and translational regulation, and augments the field of Potyviridae translation. |
