EVIDENCE THAT HIV-1 REV DIRECTLY PROMOTES THE NUCLEAR EXPORT OF UNSPLICED RNA

The Rev trans-activator of human immmunodeficiency virus type 1 (HIV-1 ) is a protein that regulates the simultaneous appearance in the cytop lasm of both spliced and unspliced forms of viral mRNAs from the same viral transcripts by way of recognition of a target sequence termed th e Rev-responsive element (RRE). Whether Rev acts directly on RNA expor t or by inhibition of splicing, or both, is still a matter of debate. We have addressed this issue in Xenopus laevis oocytes by microinjecti ng RNA molecules containing RRE along with purified recombinant Rev pr otein into the oocyte nuclei. Adenovirus pre-mRNA containing an RRE in the intron was spliced equally well in the absence and presence of Re v protein. Only in the presence of Rev was non-spliced pre-mRNA export ed from the nucleus; more surprisingly, the excised intron lariat (con taining RRE) was also exported. Furthermore, an RRE-containing mRNA mo lecule that lacked intron sequences was also efficiently exported from the nucleus in a Rev-dependent manner. Therefore our results demonstr ate that Rev can act directly at the level of nuclear export, independ ent of any inhibitory effect that it may exert on the splicing of pre- mRNA. Finally, our finding that the Rev mutant M10, shown previously t o be inactive in human lymphoid cells, was also unable to export RRE-c ontaining RNA molecules from oocyte nuclei suggests that one or more c ellular factors, evolutionarily conserved between humans and Xenopus, interact with Rev in both cell systems to promote nuclear RNA export.