Co-transcriptional recruitment of the U1 snRNP

It is currently believed that the splicing of most pre-mRNAs occurs, at least in part, co-transcriptionally. In order to validate this principle in yeast and establish an experimental system for monitoring spliceosome assembly in vivo, I have employed the chromatin immunoprecipitation (ChIP) assay to study co-transcriptional splicing events. Here, I use ChIP to examine key questions with respect to the recent proposal that RNA polymerase II (Pol II) recruits pre-mRNA splicing factors to active genes. In my thesis, I address: 1) whether the U1 snRNP, which binds to the 5¡¦ splice site of each intron, is recruited co-transcriptionally in vivo and 2) if so, where along the length of active genes the U1 snRNP is concentrated. U1 snRNP accumulates on downstream positions of genes containing introns but not within promoter regions or along intronless genes. More specifically, accumulation correlated with the presence and position of the intron, indicating that the intron is necessary for co-transcriptional U1 snRNP recruitment and/or retention (Kotovic et al., 2003). In contrast to capping enzymes, which bind directly to Pol II (Komarnitsky et al., 2000; Schroeder et al., 2000), the U1 snRNP is poorly detected in promoter regions, except in genes harboring promoter-proximal introns. Detection of the U1 snRNP is dependent on RNA synthesis and is abolished by intron removal. Microarray data reveals that intron-containing genes are preferentially selected by ChIP with the U1 snRNP furthermore indicating recruitment specificity to introns. Because U1 snRNP levels decrease on downstream regions of intron-containing genes with long second exons, our lab is expanding the study to 3¡¦ splice site factors in hopes to address co-transcriptional splicing. In my thesis, I also focus on questions pertaining to the requirements for recruitment of the U1 snRNP to sites of transcription. To test the proposal that the cap-binding complex (CBC) promotes U1 snRNP recognition of the 5¡¦ splice site (Colot et al., 1996), I use a ?´CBC mutant strain and determine U1 snRNP accumulation by ChIP. Surprisingly, lack of the CBC has no effect on U1 snRNP recruitment. The U1 snRNP component Prp40p has been identified as playing a pivotal role in not only cross-intron bridging (Abovich and Rosbash, 1997), but also as a link between Pol II transcription and splicing factor recruitment (Morris and Greenleaf, 2000). My data shows that Prp40p recruitment mirrors that of other U1 snRNP proteins, in that it is not detected on promoter regions, suggesting that Prp40p does not constitutively bind the phosphorylated C-terminal domain (CTD) of Pol II as previously proposed. This physical link between Pol II transcription and splicing factor recruitment is further tested in Prp40p mutant strains, in which U1 snRNP is detected at normal levels. Therefore, U1 snRNP recruitment to transcription units is not dependent on Prp40p activity. My data indicates that co-transcriptional U1 snRNP recruitment is not dependent on the CBC or Prp40p and that any effects of these players on spliceosome assembly must be reflected in later spliceosome events. My data contrasts the proposed transcription factory model in which Pol II plays a central role in the recruitment of mRNA processing factors to TUs. According to my data, splicing factor recruitment acts differently than capping enzyme and 3¡¦ end processing factor recruitment; U1 snRNP does not accumulate at promoter regions of intron-containing genes or on intronless genes rather, accumulation is based on the synthesis of the intron. These experiments have lead me to propose a kinetic model with respect to the recruitment of splicing factors to active genes. In this model, U1 snRNP accumulation at the 5¡¦ splice site requires a highly dynamic web of protein-protein and protein-RNA interactions to occur, ultimately leading to the recruitment and/or stabilization of the U1 snRNP