Lecture Summary
1. Basal Transcription Complex - protein complex which binds to promoter to initiate transcription. Consists of RNA polymerase PLUS (in eukaryotes) additional factors

2. Basal Transcription Complex in Prokaryotes

• Prokaryotic RNA Polymerase:

• Consists of 2a, 1b, 1b’ subunits and a s factor
• s⁷⁰ is the common one. s³² (heat shock promoters), s⁵⁴ (nitrogen regulation).
• s factor disassociates after 10 nucleotides.

3. Basal Transcription Complex in Eukaryotes

• RNA Polymerase
• Basal Factors
• General Factors

• RNA polymerase - large and complex, many subunits.

3 different polymerases - PolI, PolII, PolIII.

Some of the subunits are common to all:

 
 

• Basal Transcription Factors:  eg in PolII
• TFIID - consists of TATA-binding protein (TBP), and TAFs (including a histone acetylase).


• TBP - A universal transcription factor

Assembly of RNA PolII Complex:

• Assembly - TBP binds first, then other factors
• Also requires other Transcription Factors: TFIIA, TFIIB, TFIIF etc.
• THEN  RNA polymerase II is recruited - TFIIF already bound to  Pol II:

 

 
 
 
 
 
 
 

 

Animation of Basal Transcription Complex Assembly

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• TBP also plays a role in Pol I and Pol III-mediated transcription. eg for Pol III genes:

• General Factors - needed to give significant levels of transcription
• Sp1 - binds Sp1 box (GGGCGG)
• CCAAT Box Binding Factor (CBF) binds CCAAT Box
• Inhibition of CBF-binding can prevent transcription in some tissues:

4. Evolutionary Conservation of Transcription

•  TATA-Box present in typical prokaryotic promoter; also present in PolII and some PolIII promoters.
•  Some similarity between a large subunit of eukaryotic polymerases  and b and b’ subunit of prokaryotes.
•  Also prokaryotic a subunit is similar to another eukaryotic subunit.
•  Similarity between DNA-binding region of s70 and TBP.

Reading
Lodish - Chapter 10 p349; 361; 380-384
Instant Notes in Mol Biology 2nd Edition: K2 - p186; L3 - p205; M1 - p209; M5 - p221

Website:http://www.rockefeller.edu/labheads/darst/structures.htm
                     (models of RNA polymerases)

Questions
1. Describe how the basal transcription complex assembles to initiate transcription in eukaryotes
2. Outline the evidence which suggests that the eukaryotic transcription machinery evolved from prokaryotes

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