RNA polymerase II targets pre-mRNA splicing factors to transcription sites in vivo.

Biochemical evidence indicates that pre-mRNA splicing factors physically interact with the C-terminal domain of the largest subunit of RNA polymerase II. We have investigated the in vivo function of this interaction. In mammalian cells, truncation of the CTD of RNA pol II LS prevents the targeting of the splicing machinery to a transcription site. In the absence of the CTD, pre-mRNA splicing is severely reduced. The presence of unspliced RNA alone is not sufficient for the accumulation of splicing factors at the transcription site, nor for its efficient splicing. Our results demonstrate a critical role for the CTD of RNA pol II LS in the intranuclear targeting of splicing factors to transcription sites in vivo.