Identification and analysis of the site of -1 ribosomal frameshifting in red clover necrotic mosaic virus.

The genomic RNA-1 of red clover necrotic mosaic dianthovirus (RCNMV) contains the heptanucleotide GGAUUUU that precedes the termination codon of the 5' proximal p27 open reading frame (ORF). This heptanucleotide is followed by a sequence with the potential to form a stable, complex secondary structure. Translation of RNA-1 is postulated to utilize a -1 ribosomal frameshifting mechanism to express the 88-kDa viral RNA polymerase. Using site-directed mutagenesis together with cell-free translation to monitor frameshifting and a biological assay of the mutants in plants, we establish the role of the GGAUUUU as the site where -1 ribosomal frameshifting occurs. The frameshifting signal sequence conforms to the simultaneous slippage model. Stop codons flanking the shifty signal are not required for frameshifting but the p27 ORF termination codon is necessary for maintaining optimal infectivity of the virus. Mutations abolishing the RCNMV RNA-1 internal p57 ORF initiation codon did not affect infectivity of the virus, suggesting that this cistron is only expressed in vivo as an 88-kDa ribosomal frameshifting product. Shifty heptanucleotide signals from a number of animal retroviruses and RNA plant viruses facilitate RCNMV frameshifting in vitro. However, only a limited number of the heterologous shifty heptanucleotides were functional in plant cells. We suggest that specific shifty tRNA populations in the cell facilitate viral -1 ribosomal frameshifting. This analysis also suggests that the slippery sequence requirements are not identical in mammalian and in plant systems.