Dynamics of TFIIH and Spt5 during the transition from transcription initiation to elongation

The transition of RNA polymerase II from initiation to elongation requires the timely exchange of many factors. Some initiation factors and elongation factors contact the same RNA polymerase II surfaces in an apparently mutually exclusive manner. For example, space occupied by the initiation factor TFIIH and elongation factor Spt4/5 (DSIF) are predicted to overlap. To determine whether Spt4/5 actively displaces TFIIH, or binds only after TFIIH dissociates, labelled yeast nuclear extracts were combined with single-molecule fluorescence microscopy to observe their dynamics in real time. TFIIH categorically dissociates before Spt4/5 arrives, with a mean interval of ∼35 seconds. These results suggest that initiation complex disassembly precedes elongation complex formation.