A CIS-ACTING REPRESSIVE SEQUENCE THAT OVERLAPS THE REV-RESPONSIVE ELEMENT OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 REGULATES NUCLEAR RETENTION OF ENV MESSENGER-RNAS INDEPENDENTLY OF KNOWN SPLICE SIGNALS

The Rev protein of human immunodeficiency virus type 1 (HIV-1) binds to an RNA structure, the Rev-responsive element (RRE), to enhance expression of the viral structural genes by relieving the nuclear sequestration of incompletely spliced viral transcripts. It has been suggested that nuclear retention of these mRNAs, in mammalian cells, is due to the activity of either cis-acting repressive sequence elements or to inefficient splicing signals. Expression of the HIV-1 envelope gene in transfected Drosophila cells is also dependent upon Rev coexpression and, hence, the mechanism of nuclear retention and Rev regulation are highly conserved. Here we use the Drosophila system to identify a major cis-acting repressive sequence element that overlaps the RRE and is responsible for the nuclear entrapment and Rev-dependent expression of HIV-1 env mRNAs. Moreover, the splice signals spanning env are not required for nuclear retention or Rev-dependent trans-activation of env mRNAs. We suggest that the RRE and its associated RNA structure are necessary for both the repressive and known trans-activation effects of Rev regulation.