Trans-Acting Factors Affecting Retroviral Recoding

The production of retroviral enzymes requires a translational recoding event which subverts normal decoding, either by direct suppression of termination with the insertion of an amino acid at a stop codon (readthrough), or by an alteration of the reading frame of the mRNA (frameshift). It has been determined that retroviral readthrough and frameshift require cis-acting factors in the mRNA to stimulate recoding on the eukaryotic ribosome. Here we investigate the affects of trans acting factors on recoding, primarily in the context of the MoMLV gag-pol junction. We report the effects of a host protein, Large Ribosomal Protein Four (RPL4), on the efficiency of recoding. Using a dual luciferase reporter assay, we show that transfection of cells with an RPL4 cDNA expression construct enhances recoding efficiency in a dose-dependent manner. The increase in the frequency of recoding can be more than 2-fold, adequate to disrupt normal viral production. This effect is cell line specific, and appears to be distinct to RPL4 among ribosomal proteins. The RPL4 increase occurs with both retroviral readthrough and frameshift sequences, and even at other viral readthrough regions that do not involve RNA secondary structures. We show that RPL4 effects are negated by release factor over-expression, and that RPL4 will increase readthrough above the levels of a hyperactive mutant and in addition to G418. When cotransfected with Moloney murine leukemia provirus, the RPL4-mediated increase in readthrough reduces the amount of virus released. We also examined the effects of aminoglycoside drugs and the small molecule PTC124 on readthrough of the MoMLV gag-pol junction. We show that G418, paromomycin and PTC124 increase readthrough of our MoMLV reporter in a dose dependent manner in 293A cells. These drugs reduce viral replication, as measured by a recombinant transducing virus assay. We further examine G418 and paromomycin in an in-vitro system; readthrough is increased to higher levels than those seen in vivo. G418 displays deleterious effects on cell viability and overall translation. Paromomycin does not appear as toxic, suggesting differences in interactions by which these drugs enhance readthrough.