Expression of retroviral replication enzymes (Pol) requires a controlled 
translational recoding event to bypass the stop codon at the end of gag. This 
recoding event occurs either by direct suppression of termination via the 
insertion of an amino acid at the stop codon (readthrough) or by alteration of 
the mRNA reading frame (frameshift). Here we report the effects of a host 
protein, large ribosomal protein 4 (RPL4), on the efficiency of recoding. Using 
a dual luciferase reporter assay, we found that transfection of cells with a 
plasmid encoding RPL4 cDNA increases recoding efficiency in a dose-dependent 
manner, with a maximal enhancement of nearly twofold. Expression of RPL4 
increases recoding of reporters containing retroviral readthrough and frameshift 
sequences, as well as the Sindbis virus leaky termination signal. RPL4-induced 
enhancement of recoding is cell line specific and appears to be specific to RPL4 
among ribosomal proteins. Cotransfection of RPL4 cDNA with Moloney murine 
leukemia proviral DNA results in Gag processing defects and a reduction of viral 
particle formation, presumably caused by the RPL4-dependent alteration of the 
Gag-to-Gag-Pol ratio required for virion assembly and release.