Spatiotemporal coordination of transcription preinitiation complex assembly in live cells

Transcription initiation by RNA polymerase II (RNA Pol II) requires preinitiation complex (PIC) assembly at gene promoters. In the dynamic nucleus, where thousands of promoters are broadly distributed in chromatin, it is unclear how multiple individual components converge on any target to establish the PIC. Here we use live-cell, single-molecule tracking in S. cerevisiae to visualize constrained exploration of the nucleoplasm by PIC components and Mediator's key role in guiding this process. On chromatin, TFIID/TATA-binding protein (TBP), Mediator, and RNA Pol II instruct assembly of a short-lived PIC, which occurs infrequently but efficiently within a few seconds on average. Moreover, PIC exclusion by nucleosome encroachment underscores regulated promoter accessibility by chromatin remodeling. Thus, coordinated nuclear exploration and recruitment to accessible targets underlies dynamic PIC establishment in yeast. Our study provides a global spatiotemporal model for transcription initiation in live cells.

Automated summary

The study reveals that transcription initiation by RNA Pol II at gene promoters requires formation of the PIC, with a key role played by the Mediator complex. On chromatin, TFIID/TBP, Mediator, and RNA Pol II efficiently assemble into a short-lived PIC within seconds. Nucleosome exclusion of PIC highlights the importance of chromatin remodeling in regulating promoter accessibility.