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J. Virol., 06 1996, 3834-3843, Vol 70, No. 6
RA Ogert, LH Lee and KL Beemon
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 into viral
particles. The complex retroviruses encode Rev or Rex regulatory proteins,
which interact with cis-acting viral sequences to promote cytoplasmic
expression of incompletely spliced viral RNAs. Since the simple
retroviruses do not encode regulatory proteins, we proposed that they might
contain cis-acting sequences that could interact with cellular Rev-like
proteins. To test this possibility, we initially looked for a cis-acting
sequence in avian retroviruses that could substitute for Rev and the Rev
response element in human immunodeficiency virus type 1 expression
constructs. A cis-acting element in the 3' untranslated region of Rous
sarcoma virus (RSV) RNA was found to promote Rev-independent expression of
human immunodeficiency virus type 1 Gag proteins. This element was mapped
between RSV nucleotides 8770 and 8925 and includes one copy of the direct
repeat (DR) sequences flanking the RSV src gene; similar activity was
observed for the upstream DR. To address the function of this element in
RSV, both copies of the DR sequence were deleted. Subsequently, each DR
sequence was inserted separately back into this deleted construct. While
the viral construct lacking both DR sequences failed to replicate,
constructs containing either the upstream or downstream DR replicated well.
In the absence of both DRs, Gag protein levels were severely diminished and
cytoplasmic levels of unspliced viral RNA were significantly reduced;
replacement of either DR sequence led to normal levels of Gag protein and
cytoplasmic unspliced RNA.
Copyright © 1996, American Society for Microbiology
Avian retroviral RNA element promotes unspliced RNA accumulation in the cytoplasm
Department of Biology, John Hopkins University, Baltimore, Maryland 21218, USA.
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