AUTOGENOUS REGULATION OF RNA TRANSLATION AND PACKAGING BY ROUS-SARCOMA VIRUS PR76(GAG)

Unspliced cytoplasmic retroviral RNA in chronically infected cells eit her is encapsidated by Gag proteins in the manufacture of virus or is used to direct synthesis of Gag proteins. Several models have been sug gested to explain the sorting of viral RNA for these two purposes. Her e we present evidence supporting a simple biochemical mechanism that a ccounts for the rooting of retroviral RNA. Our results indicate that r ibosomes compete with the Gag proteins to determine the fate of nascen t retroviral RNA. Although the integrity of the entire Rous sarcoma vi rus leader sequence is important for retroviral packaging and translat ion, the RNA structure around the third small open reading frame, whic h neighbors the Psi site required for packaging of the RNA, is particu larly critical for maintenance of the balance between translation and packaging. These results support the hypothesis that Gag proteins auto genously regulate their synthesis and encapsidation of retroviral RNA and that an equilibrium exists between RNA destined for translation an d packaging that is based on the intracellular levels of Gag proteins and ribosomes. To test the model, mRNAs with natural or mutated 5' lea der sequences from Rous sarcoma virus were expressed in avian cells in the presence and absence of Pr76(gag). We demonstrate that Pr76(gag) acts as a translational repressor of these mRNAs in a dose-dependent m anner, supporting the hypothesis that Pr76(gag) can sort retroviral RN A for translation and encapsidation.