RNA processing events that take place on the transcribed pre-mRNA include 
capping, splicing, editing, 3' processing, and polyadenylation. Most of these 
processes occur co-transcriptionally while the RNA polymerase II (Pol II) enzyme 
is engaged in transcriptional elongation. How Pol II elongation rates are 
influenced by splicing is not well understood. We generated a family of 
inducible gene constructs containing increasing numbers of introns and exons, 
which were stably integrated in human cells to serve as actively transcribing 
gene loci. By monitoring the association of the transcription and splicing 
machineries on these genes in vivo, we showed that only U1 snRNP localized to 
the intronless gene, consistent with a splicing-independent role for U1 snRNP in 
transcription. In contrast, all snRNPs accumulated on intron-containing genes, 
and increasing the number of introns increased the amount of spliceosome 
components recruited. This indicates that nascent RNA can assemble multiple 
spliceosomes simultaneously. Kinetic measurements of Pol II elongation in vivo, 
Pol II ChIP, as well as use of Spliceostatin and Meayamycin splicing inhibitors 
showed that polymerase elongation rates were uncoupled from ongoing splicing. 
This study shows that transcription elongation kinetics proceed independently of 
splicing at the model genes studied here. Surprisingly, retention of 
polyadenylated mRNA was detected at the transcription site after transcription 
termination. This suggests that the polymerase is released from chromatin prior 
to the completion of splicing, and the pre-mRNA is post-transcriptionally 
processed while still tethered to chromatin near the gene end.