Wide spread and efficient co-transcriptional splicing facilitated by multiple exons

Pre-mRNA splicing is important for gene expression in most eukaryotic organisms. Regulation occurs during pre-mRNA splicing greatly expanded the transcriptome complexity. Recent studies from Mammals, Drosophila and Yeast showed that the majority of introns are spliced co-transcriptionally. However, in plant the nature of co-transcriptionally splicing (CTS) and its regulation is still largely unknown. Here, through sequencing the chromatin-bound RNA (CB-RNA-seq), we studied the feature of CTS in Arabidopsis. We found CTS is widespread in Arabidopsis seedlings and a large proportion of alternative splicing events are determined co-transcriptionally. We found the CTS efficiency correlate with gene expression level, chromatin landscape and most surprisingly, the number of intron/exon of individual genes, whilst independent of gene length. In combination with iCLIP analysis, we found splicing regulator RZ-1B/1C promotes efficient CTS of thousands genes involving direct binding mainly to the exonic sequences. Interestingly, for many cases, the splicing promotion activity of RZ-1C does not associate with its binding to the regions immediately adjacent to the regulated intron. We propose a model of plant gene splicing, where multiple exon of individual gene is in favour of efficient CTS likely involving RZ-1C cooperative interactions with many exons and splicing factors. Our work uncovers the robustness of plant CTS and highlighted the role of RZ-1C in this process. chromatin-bound RNA sequencing and iCLIP-seq were used to investigate the basic feature and regulation of co-transcriptional pre-mRNA splicing