Kinetic analysis shows that RNA polymerase elongation kinetics are not modulated by co-transcriptional splicing and that post-transcriptional splicing can proceed at the site of transcription without the presence of the polymerase.

Here, we explore the role of splicing in transcription, employing both genome-wide analysis of human ChIP-seq data and experimental manipulation of exon-intron organization in transgenic cell lines. We show that the activating histone modifications H3K4me3 and H3K9ac map specifically to first exon-intron boundaries. This is surprising, because these marks help recruit general transcription factors (GTFs) to promoters. In genes with long first exons, promoter-proximal levels of H3K4me3 and H3K9ac are greatly reduced; consequently, GTFs and RNA polymerase II are low at transcription start sites (TSSs) and exhibit a second, promoter-distal peak from which transcription also initiates. In contrast, short first exons lead to increased H3K4me3 and H3K9ac at promoters, higher expression levels, accuracy in TSS usage, and a lower frequency of antisense transcription. Therefore, first exon length is predictive for gene activity. Finally, splicing inhibition and intron deletion reduce H3K4me3 levels and transcriptional output. Thus, gene architecture and splicing determines transcription quantity and quality as well as chromatin signatures.

No abstract

Many nascent long non-coding RNAs (lncRNAs) undergo the same maturation steps as pre-mRNAs of protein-coding genes (PCGs), but they are often poorly spliced. To identify the underlying mechanisms for this phenomenon, we searched for putative splicing inhibitory sequences using the ncRNA-a2 as a model. Genome-wide analyses of intergenic lncRNAs (lincRNAs) revealed that lincRNA splicing efficiency positively correlates with 5′ss strength while no such correlation was identified for PCGs. In addition, efficiently spliced lincRNAs have higher thymidine content in the polypyrimidine tract (PPT) compared to efficiently spliced PCGs. Using model lincRNAs, we provide experimental evidence that strengthening the 5′ss and increasing the T content in PPT significantly enhances lincRNA splicing. We further showed that lincRNA exons contain less putative binding sites for SR proteins. To map binding of SR proteins to lincRNAs, we performed iCLIP with SRSF2, SRSF5 and SRSF6 and analyzed eCLIP data for SRSF1, SRSF7 and SRSF9. All examined SR proteins bind lincRNA exons to a much lower extent than expression-matched PCGs. We propose that lincRNAs lack the cooperative interaction network that enhances splicing, which renders their splicing outcome more dependent on the optimality of splice sites.