HYPERSENSITIVE SPOTS AND

ENHANCERS & BLOCKERS OF RNA

SYNTHESIS

By

Monisha Jayabalan

Ist MSc medical biochemistry

Dr.A.L.Mudaliar postgraduate institute of basic medical sciences, University of Madras.

• hypersensitive site is a short region of chromatin and is

detected by its super sensitivity to cleavage by DNase I

and other various nucleases (DNase II and micrococcal

nucleases).

• the nucleosomal structure is less compacted.

• hypersensitive sites are found only in gene is being

expressed, and do not occur when the gene is inactive.

• Hypersensitive sites are generated as a result of the

binding of transcription factors that displace histone

octamers

ENHANCERS• Enhancer sequences are regulatory DNA sequences that,

when bound by specific proteins called transcription

factors, enhance the transcription of an associated gene.

• Enhancer is a short region of about 50-1500 bp.

• Transcription factors can bind to enhancer sequences

located upstream or downstream from an associated

gene.

• enhancer sequences can be located thousands of base

pairs away from the transcription start site of the gene

being regulated.

• They are cis- acting

• It cannot act on the promoter region all by itself but by

binding with activator protein

• Enhancers can also be found within introns.

Enhanceosome• higher-order protein complex assembled at the enhancer

and regulating expression of a target gene

• The binding and assembly of the activating proteins is

cooperative due to energetically favourable protein-

protein interactions formed in the complex.

Example : human interferon-beta gene

• Three activator proteins - NF-κB, an interferon activator

protein such as IRF-3, and the ATF-2/c-Jun complex -

cooperatively bind to the upstream enhancer region upon

viral infection.

• The interaction is mediated by a fourth protein HMG-I

BLOCKERSActinomycin D:

• elongation of RNA strands by RNA polymerase in bacteria

and eukaryotic RNA polymerases inhibitor.

• The planar portion of this molecule inserts (intercalates)

into the double helical DNA between successive G C base

pairs, deforming the DNA.

• Arcidine

Rifampicin

inhibits bacterial RNA synthesis by binding to the β subunit

of bacterial RNA polymerases, preventing the promoter

clearance step of transcription

• α-amanitin

• disrupts mRNA formation in animal cells by blocking Pol II

and, at higher concentrations Pol III.

• this is a toxic substance secreted by Amanita phalloides