Translational frameshifting mediated by a viral sequence in plant cells.

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Abstract
It has been proposed that the polymerase gene of barley yellow dwarf virus and related viruses is expressed by a ribosomal frameshift event during translation. The 5' end of this gene overlaps with the 3' end of an upstream gene that is in a different reading frame. The region of overlap is similar to sequences in retro- and coronaviruses that are known to express their polymerase genes by frameshifting. This overlap region includes a "shifty" heptanucleotide, followed by a highly structured region that may contain a pseudoknot. Sequences of 115 or 144 base pairs that span this region from barley yellow dwarf virus (PAV serotype) genomic RNA were introduced into a plasmid, so that a reporter gene could be expressed in plant cells only if a minus one (-1) frameshift event occurred. Frameshifting was detected at a rate of approximately 1%. This frameshifting was abolished when the stop codon at the 3' end of the upstream open reading frame was deleted. A sequence expected to form a strong stem-loop immediately upstream of the frameshift site was unnecessary for frameshifting, and initiation at AUG codons within the stem-loop appeared to be inhibited. Like viruses that infect hosts in other kingdoms, plant viruses also can induce frameshifting in translation of their genes.