Eukaryotic Transcription: Overview/How RNA pols do knows where to start copying the DNA?/Promoter Accessibility: Role of Nucleosome

 

Eukaryotic Transcription: Overview

·  Separate polymerase for the synthesis of rRNA, tRNA, mRNA

·       Initiation

i. Recognition of promoter DNA

ii. Preinitiation complex (closed complex)

iii. DNA opening (Open complex)

iv. Synthesis of a short initial RNA oligomer

·  Elongation

i. Polymerase uses the DNA template to extend the growing RNA chain in a processive manner

·    Termination

i. Release of RNA

ii. Recycling of RNA polymerase

·  The eukaryotic RNA polymerase is unable to accomplish the transcription cycle alone.

Eukaryotic transcription Requirements

·  The eukaryotic RNA polymerase is unable to accomplish the transcription cycle alone. Thus it

requires some additional factors.

 

Nuclear RNA polymerase

Three distinct types of RNA pol are required for the synthesis of all the RNAs. All are large enzyme with multiple subunits.

RNA pol II

·         Multiprotein complex

·         Interacts with transcription factor to form PIC

·         Contains a carboxy-terminal domain (CTD)

·         Activated by phosphorylation of serine and threonine residue in the CTD

·         CTD phosphorylation/dephosphorylation is critical for transcription

·         Kinase subunit of TFIIH phosphorylates CTD

Promoters for RNA pol II

i. Hogness box

·         Centered about ~25 bases upstream to the transcription start site

·         Contain TATAAA sequence of nucleotides

·         Also called TATA box

Core promoter elements: TATA, Inr, DPE

Proximal regulatory elements: Those sequences lie close to the core promoter (GC and CAAT boxes)

Distal regulatory elements: Those sequences farther away to the core promoter (Enhancer element)

General transcription factors (GTF)

·         GTF are known as TFII (transcription factor for RNA pol II)

·         Binds to the promoter with RNA pol II and forms a pre-initiation complex (PIC)

·         TFIID=TBP+TAF(13-14 factors) I^st transcription factor binds to promoter region

§  TFIID constitutes  TBP and TAF

§  TBP (TATA box binding protein and recognizing the TATA box)

§  TAF (TATA box binding protein associated factor)

·         TFIIA 2^nd TF that stabilizes binding of TBP to the promoter

·         TFIIB 3^rd TF recruits RNA pol II-TFIIF complex

·       TFIIF 4^th TF binds tightly to RNA pol II TFIIB and prevents binding of RNA pol II to non-specific DNA

·         TFIIE 5^th TF recruits TFIIH, stimulate ATPase and helicase activities of TFIIH

·      TFIIH 6^th TF unwinds DNA at promoter site (helicase activity)

§  Phosphorylates CTD (ATPase activity)

§  Recruits nucleotide excision repair proteins

Regulatory elements and transcriptional activators

Additional consensus sequences lie upstream of the core promoter region. STF (specific transcription factors) or activators and repressors binds to these elements and regulate transcription.

How RNA pols do knows where to start copying the DNA?

Promoter Accessibility: Role of Nucleosome

Heterochromatin: Compact and transcriptionally inactive

Euchromatin: Open and transcriptionally active

Chromatin Remodeling OR Covalent modification of histones

Acetylation of lysine residues at the amino terminus of histones by histone acetyltransferase (HAT)

Deacetylation by histone deacetyl transferase (HDAT)