about hyersensitive spots in DNA and also about enhacers and blockers of RNA synthesis

1. 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.

2. • 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.

3. • 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

4. 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

5. • It cannot act on the promoter region all by itself but by
binding with activator protein
• Enhancers can also be found within introns.

7. 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.

8. 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

10. BLOCKERS
Actinomycin 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.

11. • Arcidine

12. Rifampicin
inhibits bacterial RNA synthesis by binding to the β subunit
of bacterial RNA polymerases, preventing the promoter
clearance step of transcription

14. • 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