Schematic illustration of the effect of splicing on DNA topology and accessibility to RNA polymerase II. (A) SR proteins inhibit the cleavage and religation activity of DNA topoisomerase I (Top1). Therefore, after one round of transcription, DNA becomes less twisted and more accessible to RNA polymerase II (RNAP II). (B) In an intronless gene, Top1 actively removes the negative supercoiling generated by transcription. Transcription does not change the topological status of an intronless gene. For simplicity, nucleosomes are not shown.