Ribosomal frameshifting directed by a potyvirid sequence motif in diverse translation systems
The family, Potyviridae, contains a third of all known plant viruses within eight genera. The genome is translated as one large polyprotein from which ten mature proteins are proteolytically cleaved. Recently, discovery of a protein translated from an overlapping reading frame (pipo) proved essential in systemic plant infection. It is hypothesized to translate via a ribosomal frameshift, change in reading frame during the elongation phase of translation. The cryptic programmed ribosomal frameshift signal had been undetected until now because it lacks the canonical characteristics: slippery sequence, spacer sequence, and RNA secondary structure, in that order. Only one of the three components, the slippery sequence, is present as a conserved eight base G1-2A6-7 motif at the 5' end of pipo. Using Turnip mosaic virus, sequence surrounding the conserved putative frameshift signal was tested for efficiency of shifting in a dual luciferase cassette where the expression of the downstream firefly luciferase requires a -1/+2 change in frame. The conserved G_GAA_AAA_A sequence is inherently slippery with similar efficiency as other stretches of a single base, i.e. AAA_AAA_A. An added benefit is present when G2A6 is surrounded by viral sequence. This suggests a positively acting cis element like RNA secondary structure, but deletion analysis could not detect any specific elements. Also, addition of RNA secondary structure did not increase frameshift efficiency; rather it had minimal impact on the relative rate to the levels of G2A6 by itself. Sequence analysis will assist in identifying the directionality of the frameshift. This is a new signal for programmed -1 ribosomal frameshifting, or it could be the first +2 frameshift signal to be reported.