Date of this Version
Roberts R, Zhang J, Mayberry LK, Tatineni S, Browning KS, Rakotondrafara AM. 2015. A unique 5= translation element discovered in triticum mosaic virus. J Virol 89:12427–12440. doi:10.1128/JVI.02099-15.
Several plant viruses encode elements at the 5' end of their RNAs, which, unlike most cellular mRNAs, can initiate translation in the absence of a 5' m7GpppG cap. Here, we describe an exceptionally long (739-nucleotide [nt]) leader sequence in triticum mosaic virus (TriMV), a recently emerged wheat pathogen that belongs to the Potyviridae family of positive-strand RNA viruses. We demonstrate that the TriMV 5' leader drives strong cap-independent translation in both wheat germ extract and oat protoplasts through a novel, noncanonical translation mechanism. Translation preferentially initiates at the 13th start codon within the leader sequence independently of eIF4E but involves eIF4G. We truncated the 5= leader to a 300-nucleotide sequence that drives cap-independent translation from the 5' end. We show that within this sequence, translation activity relies on a stem-loop structure identified at nucleotide positions 469 to 490. The disruption of the stem significantly impairs the function of the 5' untranslated region (UTR) in driving translation and competing against a capped RNA. Additionally, the TriMV 5' UTR can direct translation from an internal position of a bicistronic mRNA, and unlike cap-driven translation, it is unimpaired when the 5' end is blocked by a strong hairpin in a monocistronic reporter. However, the disruption of the identified stem structure eliminates such a translational advantage. Our results reveal a potent and uniquely controlled translation enhancer that may provide new insights into mechanisms of plant virus translational regulation.