RNA Pol II Length and Disorder Enable Cooperative Scaling of Transcriptional Bursting

RNA Polymerase II contains a disordered C-terminal domain (CTD) whose length enigmatically correlates with genome size. The CTD is crucial to eukaryotic transcription, yet the functional and evolutionary relevance of this variation remains unclear. Here, we use smFISH, live imaging, and RNA-seq to investigate how CTD length and disorder influence transcription. We find that length modulates the size and frequency of transcriptional bursting. Disorder is highly conserved and mediates CTD-CTD interactions, an ability we show is separable from protein sequence and necessary for efficient transcription. We build a data-driven quantitative model, simulations of which recapitulate experiments and support CTD length promotes initial polymerase recruitment to the promoter but slows down its release from it, and that CTD-CTD interactions enable promoter recruitment of multiple polymerases. Our results reveal how these tunable parameters provide access to a range of transcriptional activity, offering a new perspective for the mechanistic significance of CTD length and disorder in transcription across eukaryotes. Overall design: We measured the transcriptional phenotype of CTD mutants and their response to galactose using RNA-seq, comparing wild-type with strains of 10 CTD repeats (CTDr), 10CTDr fused to FUS low complexity domain (LCD), and 10CTDr fused to TAF LCD.