Antagonistic effects of T-Ag and VP16 reveal a role for RNA pol II elongation on alternative splicing

Here we investigate the promoter control of alternative splicing by studying two transcriptional activators on templates under replicating conditions. SV40 large T‐antigen (T‐Ag) activates template replication only 2‐fold but transcription 25‐fold. T‐Ag‐mediated replication, reported to inhibit RNA polymerase II elongation, provokes a 10‐ to 30‐fold increase in the inclusion of the fibronectin EDI exon into mature mRNA. The T‐Ag effect is exon specific, occurs in cis and depends strictly on DNA replication and not on cell transformation. VP16, an activator of transcriptional initiation and elongation, has a similar effect on transcription but the opposite effect on splicing: EDI inclusion is inhibited by 35‐fold. VP16 completely reverts the T‐Ag effect, but a VP16 mutant with reduced elongation ability provokes only partial reversion. Both T‐Ag and VP16 promote conspicuous co‐localization of mRNA with nuclear speckles that contain the SR protein SF2/ASF, a positive regulator of EDI inclusion. Therefore, we conclude that co‐localization of transcripts and speckles is not sufficient to stimulate EDI inclusion.