Review of RNA Polymerase II Holoenzymes and Subcomplexes (pdf format)
Review of RNA Polymerase II Holoenzymes and Subcomplexes (html format)
The RPB1 subunit is heavily phosphorylated in a repeated YSPTSPS amino acid sequence found in the carboxy terminal domain (CTD). This is repeated 27 times in the yeast RPB1 (less than ten repeats is lethal, more than 13 is viable). This is repeated 52 times in the mouse RPB1 equivalent protein (less than 25 repeats is lethal, more than 36 is viable).
- TFIIB: connection between pol II holoenzyme and TFIID
- TFIID: binds to TATA sequence through TBP (also contains TAFs)
- TFIIH: kinase phosphorylates CTD of pol II after PIC to allow elongation (see SRB/mediator below), has ATPase and helicase activities, required for promoter clearance.
- TFIIA is probably more like a coactivator: it can compete with negative coactivator 2 (NC2) for binding TBP.
Basal and Regulated Initiation Complexes
Model of RNA Polymerase II Transcription Initiation Machinery
SRB/Mediator
- Mediates "activated" transcription (interacts with activators and coactivators)
- kinase phosphorylates CTD of pol II before PIC to prevent transcription (see TFIIH above).
- Subgroup of negatively acting "repressors" such as Gal11, Sin4, etc.
SWI/SNF
- Chromatin remodeling: increases nucleosomal DNA to attack by DNaseI, reduces negative superhelical turns constrained by nucleosome arrays
- Promotes nucleosome mobility
- Stimulates transcription factor binding
- Has helicase and ATPase activities (Swi2)
SAGA
- Has histone acetyltransferase activity (HAT) which acetylates lysines on histones.
- Human PCAF has HAT activity and also interacts with p300/CBP, a coactivator with HAT activity. p300/CBP also binds to many transcription factors such as CREB, Jun, Fos, Myb, and MyoD, and can acetylate not only histones but transcription factors such as p53.
- Negative coactivator 2 (NC2 ) recognizes TBP-bound promoters and inhibits entry of TBIIB. This is antagonized by competitive binding of TFIIA (see above).
- MOT1 is an ATP-dependent transcription inhibitor that destabilizes the TBP-DNA complex.
- dTAF230 is the homolog of hTAF250 and yTAF130; all of which have HAT activity.