FIG. 1.

Diagram of the 392-nt splicing and PA RNA substrate (WT Sp/PA), also called MXSVL (23). It is composed of two separable cassettes, one for splicing and one for PA. Each cassette can function on its own in splicing and PA reactions, respectively. When together, they can function in coupled splicing and PA under the appropriate conditions (see text). The splicing cassette is derived from the adenovirus major late region; the 5′-SS (5′), 3′-SS (3′), and the PPT are shown. The PA cassette contains the PA signal from the SV40 late coding region, the SVLPA, SV40 nt 2531 to 2731. The structural features of the SVLPA are shown: these include the USEs, the AAUAAA, the cleavage and PA site (An), and the DSEs (see the text for details). When the substrate is spliced, polyadenylated exons 1 and 2 are joined.