Dynamic Imaging of Nascent RNA Reveals General Principles of Transcription Dynamics And Stochastic Splice Site Selection

The activities of RNA polymerase and the spliceosome are responsible for the heterogeneity in the abundance and isoform composition of mRNA in human cells. However, the dynamics of these megadalton enzymatic complexes working in concert on endogenous genes have not been described. Here, we establish a quasi-genome-scale platform for observing synthesis and processing kinetics of single nascent RNA molecules in real time. We find that all observed genes show transcriptional bursting. We also observe large kinetic variation in intron removal for single introns in single cells which is inconsistent with deterministic splice site selection. Transcriptome-wide footprinting of the U2AF complex and nascent RNA sequencing further reveals widespread splice site selection within introns. We propose and validate a unified theoretical model to explain the general feature of transcription and pervasive stochastic splice site selection.