FIG. 6.

Anti-Rex aptamers mediate Rex responsiveness. (A) CAT reporter system and XRE chimeras. Plasmid pCMVXRE contains a strong CMV promoter that can generate an mRNA in which the sequences of the CAT protein and the XRE are embedded between splice donor (SD) and splice acceptor (SA) sites of an HIV-1 intron. In the absence of Rex, the intron is spliced out of the mRNA and degraded and little CAT activity can be observed in cell extracts. When pCMVXRE is transiently transfected into cells along with a Rex expression plasmid (pcREX), Rex mediates cytoplasmic transport of unspliced mRNA, more CAT is translated from the intact mRNAs, and the greater amounts of CAT activity can be observed in cell extracts. The amount of CAT activity observed is roughly proportional to the number of Rex-RNA complexes that are formed in the cell. (B) CAT assay of chimeric Rex-responsive elements. Reporter plasmids containing the wild-type XRE or XRE chimeras were cotransfected into CV-1 cells in the presence (+) or absence (−) of the Rex expression plasmid pcREX (24). Forty hours posttransfection, the cells were harvested and assayed for CAT activity. The identity of the aptamer that gave rise to the XRE chimera is indicated at the left. The positions of unacetylated and acetylated chloramphenicol on the TLC plate are shown.