AVIAN RETROVIRAL RNA ELEMENT PROMOTES UNSPLICED RNA ACCUMULATION IN THE CYTOPLASM

All retroviruses need mechanisms for nucleocytoplasmic export of their unspliced RNA and for maintenance of this RNA in the cytoplasm, where it is either translated to produce Gag and Pol proteins or packaged i nto viral particles. The complex retroviruses encode Rev or Rex regula tory proteins, which interact with cis-acting viral sequences to promo te cytoplasmic expression of incompletely spliced viral RNAs. Since th e simple retroviruses do not encode regulatory proteins, we proposed t hat they might contain cis-acting sequences that could interact with c ellular Rev-like proteins, To test this possibility, we initially look ed for a cis-acting sequence in avian retroviruses that could substitu te for Rev and the Rev response element in human immunodeficiency viru s type 1 expression constructs, A cia-acting element in the 3' untrans lated region of Rous sarcoma virus (RSV) RNA was found to promote Rev- independent expression of human immunodeficiency virus type 1 Gag prot eins. This element was mapped between RSV nucleotides 8770 and 8925 an d includes one copy of the direct repeat (DR) sequences flanking the R SV arc gene; similar activity was observed for the upstream DR, To add ress the function of this element in RSV, both copies of the DR sequen ce were deleted. Subsequently, each DR sequence was inserted separatel y back into this deleted construct, While the viral construct lacking both DR sequences failed to replicate, constructs containing either th e upstream or downstream DR replicated well, In the absence of both DR s, Gag protein levels were severely diminished and cytoplasmic levels of unspliced viral RNA were significantly reduced; replacement of eith er DR sequence led to normal levels of Gag protein and cytoplasmic uns pliced RNA.