Studies of the astrovirus signal that induces (-1) ribosomal frameshifting

Human astroviruses use a (-1) ribosomal frameshift mechanism to regulate expression of the viral RNA-dependent RNA polymerase gene. To evaluate the efficiency of the astrovirus frameshift signal in cell culture, different regions of the frameshift signal were cloned into the rhesus rotavirus VP4 gene and expressed in an infection-transfection transient expression cell-culture system. The various astrovirus-VP4 constructs were transfected into BHK-21 cells infected with a recombinant vaccinia virus that expresses T7 RNA polymerase (vTF7-3). All constructs contain a T7 promoter, a picornavirus internal ribosome entry site, and a T7 terminator. Frameshifted and non-frameshifted proteins were distinguished by immunoprecipitation with monoclonal antibodies specific for either the VP4 amino- or carboxy-terminus. Frameshifting efficiency was calculated as the ratio of radioactive counts in the frameshifted protein to the total counts in both the frameshifted and nonframeshifted proteins as determined by Phosphorimager analysis. We found the efficiency of astrovirus frameshifting in intact cells to be 25-28%, significantly greater than the 5-7% efficiency reported previously in a cell-free uncoupled translation system. Since the transfected plasmid is expressed in the cytoplasm in the infection-transfection system, the frameshifting efficiency determined by this assay may be a more accurate reflection of the level of frameshifting for this RNA virus in which transcription and translation are likely coupled in the cytoplasm of infected cells. This hypothesis is supported by the observation that the level of astrovirus frameshifting is increased three-fold when evaluated in a cell-free coupled transcription-translation system. These studies also confirm in intact cells what was previously determined in cell-free studies: the shifty heptamer is an absolute requirement for astrovirus ribosomal frameshifting, but deletion of sequences downstream of the stem-loop that are potentially involved in pseudoknot formation does not affect the efficiency of frameshifting.