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Section: Genetics » Expression of Gene » Transcription in Prokaryotes and Eukaryotes
 
 
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  Single RNA polymerase in E. coli
 
     
 
Content
⇒ Expression of Gene : Protein Synthesis 2.  Transcription in Prokaryotes and Eukaryotes
⇒ Transcription in prokaryotes 
⇒ Single RNA polymerase in E. coli
⇒ Promoter sites for initiation of transcription in prokaryotes
⇒ Initiation and elongation of RNA synthesis in prokaryotes
⇒ 'Inchworm model' for elongation of transcript
⇒ Elongation arrest vs termination of transcription
⇒ Termination and antitermination of mRNA synthesis in prokaryotes
⇒ Transcription in eukaryotes 
⇒ Multiple RNA polymerases in eukaryotes
⇒ Promoter, enhancer and silencer sites for initiation of transcription in eukaryotes
⇒ Transcription factors and initiation of RNA synthesis in eukaryotes
⇒ Formation of preinitiation (transcription) complex with RNA polymerase II (Pol II)
⇒ Structure and role of TFIID and other transcription factors (TBP, TAFs)
⇒ TFIIB domains for interaction with TFIID/TATA complex
⇒ Phosphorylation of CTD of a subunit of Pol II
⇒ Formation of pre-initiation complex with Pol I and Pol III
⇒ Separate DNA binding and transcription activation domains
⇒ Transcription factors and elongation of RNA chains in eukaryotes
⇒ Chromatin structure and transcription
⇒ Transcription in mitochondria
⇒ Transcription of vertebrate mtDNA
⇒ Transcription of yeast and plant mtDNA
⇒ Transcription in chloroplasts
Transcription in Prokaryotes
Single RNA polymerase in E. coli
In bacterial systems like E. coli a single RNA polymerase (RNAP) species is responsible for synthesis of all kinds of RNAs (mRNA, tRNAs and rRNA). This RNA polymerase has been purified and its structure and function is now known in some detail. It consists of five polypeptide chains including two chains of alpha (α) polypeptide and one chain each of beta (β), beta dash (β') and sigma (σ) polypeptides (Fig. 32.2), the details for which are given in Table 32.1. The RNA polymerase molecule, thus can be represented as α2ββ'σ, in which attachment of sigma (σ) factor is not very firm, so that the core enzyme (α2ββ') can be easily isolated. The active sites of core enzyme are shown in Figure 32.3. Functions of different polypeptide chains are now understood, though not in any detail. For instance β and β', which form the 'catalytic centre' of RNAP, help RNA polymerase in unwinding of DNA molecule for transcription. The sigma (σ) factor helps in recognition of start signals on DNA molecule and directs RNA polymerase in selecting the initiation sites. In the absence of sigma (σ), core enzyme initiates RNA synthesis in a random manner, suggesting the role of σ in recognition of initiation sites. Once RNA synthesis is initiated, σ dissociates after RNA is 8-9 bases long and then the core enzyme brings about elongation of mRNA. The dissociated sigma factor may again combine with core enzyme to form RNA polymerase holoenzyme (Fig. 32.4).

A model of the structure of prokaryotic RNA polymerase showing association of five polypeptides (α2ββ').
Fig. 32.2. A model of the structure of prokaryotic RNA polymerase showing association of five polypeptides (α2ββ').
 
Active centres in bacterial RNA polymerase enzyme.
Fig. 32.3. Active centres in bacterial RNA polymerase enzyme.

Role of sigma factor Nus A, and core enzyme of RNA polymerase during transcription.
Fig. 32.4. Role of sigma factor Nus A, and core enzyme of RNA polymerase during transcription.


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The α and β' have constant sizes in most bacteria; the σ varies from 32,000 to 92,000.

Although, in prokaryotes like E. coli, all RNA synthesis is done by only one kind of RNA polymerase molecules, there may be more than one sigma (σ) factors, which associate, each with the same core enzyme at different times for expression of different genes. For example, in E. coli, besides σ70 used under normal conditions of growth, atleast three other sigma factors (σ32, σ54, σ28) are now known, which are used under adverse conditions like high temperature, nitrogen deficiency and for chemotaxis (consult Regulation of Gene Expression 1.  Operon Circuits in Bacteria and other Prokaryotes for more details).



 
     






     
     
 
 
     
 
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