Production of the structural and enzymatic proteins of type 1 human immunodeficiency virus (HIV-1) is controlled by the rev regulatory gene product. The 116-amino acid Rev protein acts by binding to the Rev response element (RRE), a complex RNA stem-loop structure located within the env gene of HIV. Rev exerts a series of posttranscriptional effects, including the inhibition of viral RNA splicing, the activation of nuclear export of incompletely spliced viral RNAs, and the enhancement of translation of RRE-containing RNAs. Our studies now demonstrate that at least one member of the SR family of splicing factors, SF2͞ASF, specifically binds to a subregion of the RRE in vitro in a Rev-dependent manner. Furthermore, expression of high levels of SF2͞ASF inhibits Rev function and impairs HIV replication in vivo. Both the in vitro binding of SF2͞ASF to the Rev͞RRE complex and the in vivo inhibition of Rev action by SF2͞ASF are abrogated by mutation of the N-terminal RNA recognition motif but are not affected by mutation of the C-terminal arginine-serinerich domain. These findings suggest that Rev inhibition of HIV splicing likely involves recruitment of the essential splicing factor SF2͞ASF to the Rev͞RRE complex. However, these inhibitory effects of Rev on viral RNA splicing are apparently overcome by augmenting the intracellular levels of SF2͞ASF expression.Rev is an essential regulatory protein of the type 1 human immunodeficiency virus (HIV-1) that controls production of proteins required for the assembly of infectious virions (1-5). The 9-kb full-length genomic transcript of HIV undergoes a complex series of posttranscriptional splicing events resulting in the generation of more than 20 mRNAs that encode 9 different viral proteins (for review see refs. 6-8). The resultant mRNAs can be grouped in three size classes: the 2-kb viral RNAs are fully spliced and encode the regulatory proteins of HIV, including Rev, Tat, and Nef, whereas the 9-and 4-kb mRNAs encode the structural, enzymatic, and ancillary viral proteins, including Gag, Pol, Vif, Vpu, Vpr, and Env. The appearance of the 4-and 9-kb classes of mRNAs in the cytoplasm is regulated by the Rev protein (1-5). Rev binds specifically to the Rev response element (RRE), a complex 220-nucleotide RNA stem-loop structure located in the env coding sequence. This binding leads to effective cytoplasmic expression of the singly spliced (4 kb) and unspliced (9 kb) viral mRNAs, thus allowing the transition from early-stage regulatory protein synthesis to late-stage structural and enzymatic viral protein production (2, 6, 7). In the absence of Rev, these incompletely spliced RRE-containing transcripts are retained in the nucleus where they are either completely spliced or degraded (6, 7). Various mechanisms have been suggested to explain the action of Rev, including inhibition of viral mRNA splicing, activation of viral RNA transport from the nucleus to the cytoplasm, and enhanced translation of RRE-containing viral RNAs (1, 6-8). These mechanisms are not mutually ex...