The RNA of maize chlorotic mottle virus - the essential virus in maize lethal necrosis disease - is translated via a panicum mosaic virus-like cap-independent translation element
Authors: Carino Elizabeth , Scheets Kay , Miller W. Allen 
Published: January 2020

Maize chlorotic mottle virus (MCMV) combines with a potyvirus in maize lethal necrosis disease (MLND), an emerging disease worldwide that often causes catastrophic yield loss. To inform resistance strategies, we characterized the translation initiation mechanism of MCMV. We report that, like other tombusvirids, MCMV RNA contains a cap-independent translation element (CITE) in its 3’ untranslated region (UTR). The MCMV 3’ CITE (MTE) was mapped to nucleotides 4164-4333 in the genomic RNA. SHAPE probing revealed that the MTE is a variant of the panicum mosaic virus-like 3’ CITE (PTE). Like the PTE, electrophoretic mobility shift assays (EMSAs) indicated that eukaryotic translation initiation factor 4E (eIF4E) binds the MTE despite the absence of a m7GpppN cap structure, which is normally required for eIF4E to bind RNA. The MTE interaction with eIF4E suggests eIF4E may be a soft target for engineered resistance to MCMV. Using a luciferase reporter system, mutagenesis to disrupt and restore base pairing revealed that the MTE interacts with the 5’ UTRs of both genomic RNA and the 3’-coterminal subgenomic RNA1 via long-distance kissing stem-loop base pairing to facilitate translation in wheat germ extract and in protoplasts. However, the MTE is a relatively weak stimulator of translation and has a weak, if any, pseudoknot, which is present in the most active PTEs. Most mutations designed to form a pseudoknot decreased translation activity. Mutations in the viral genome that reduced or restored translation prevented and restored virus replication, respectively, in maize protoplasts and in plants. We propose that MCMV, and some other positive strand RNA viruses, favors a weak translation element to allow highly efficient viral RNA synthesis.

Author Summary

In recent years, maize lethal necrosis disease has caused massive crop losses in East Africa and Ecuador. It has also emerged in East Asia. Maize chlorotic mottle virus (MCMV) infection is required for this disease. While some tolerant maize lines have been identified, there are no known resistance genes that confer full immunity to MCMV. In order to design better resistance strategies against MCMV, we focused on how the MCMV genome is translated, the first step of gene expression required for infection by all positive strand RNA viruses. We identified a structure (cap-independent translation element) in the 3’ untranslated region of the viral RNA genome that allows the virus to usurp a host translation initiation factor in a way that differs from host mRNA interactions with the translational machinery. This difference may guide engineering of – or breeding for – resistance to MCMV. Moreover, this work adds to the diversity of known eukaryotic translation initiation mechanisms, as it provides more information on mRNA structural features that permit noncanonical interaction with a translation factor. Finally, owing to the conflict between ribosomes translating and viral replicase copying viral RNA, we propose that MCMV has evolved a relatively weak translation element in order to permit highly efficient RNA synthesis, and that this replication-translation trade-off may apply to other positive strand RNA viruses.