Coupling between transcription and RNA processing is a key gene regulatory 
mechanism. Here we use chromatin immunoprecipitation to detect 
transcription-dependent accumulation of the precursor mRNA (pre-mRNA) splicing 
factors hnRNP A1, U2AF65 and U1 and U5 snRNPs on the intron-containing human FOS 
gene. These factors were poorly detected on intronless heat-shock and histone 
genes, a result that opposes direct recruitment by RNA polymerase II (Pol II) or 
the cap-binding complex in vivo. However, an observed RNA-dependent interaction 
between U2AF65 and active forms of Pol II may stabilize U2AF65 binding to 
intron-containing nascent RNA. We establish chromatin-RNA immunoprecipitation 
and show that FOS pre-mRNA is cotranscriptionally spliced. Notably, the 
topoisomerase I inhibitor camptothecin, which stalls elongating Pol II, 
increased cotranscriptional splicing factor accumulation and splicing in 
parallel. This provides direct evidence for a kinetic link between 
transcription, splicing factor recruitment and splicing catalysis.