TFIIS elongation factor and Mediator act in conjunction during transcription initiation in vivo

The transcription initiation and elongation steps of protein-coding genes usually rely on unrelated protein complexes. However, the THIS elongation factor is implicated in both processes. We found that, in the absence of the Med31 Mediator subunit, yeast cells required the THIS polymerase II (PoI II)-binding domain but not its RNA cleavage stimulatory activity that is associated with its elongation function. We also found that the THIS PoI II-interacting domain was needed for the full recruitment of PoI 11 to several promoters in the absence of Med31. This work demonstrated that, in addition to its thoroughly characterized role in transcription elongation, THIS is implicated through its PoI II-binding domain in the formation or stabilization of the transcription initiation complex in vivo.