BIO282 Lecture Notes - Lecture 18: Response Element, Dephosphorylation, Antitermination
Regulation of eukaryotic gene expression
Regulation at genome level
a. Chromatin decondensation
• Chromosomal puffs are regions of relaxed chromatin where active transcription is taking place.
It is a visual evidence that chromatin decondensation is involved in genomic control.
Chromosomal puffs on giant polytene chromosomes isolated from the salivary glands of larval
Drosophila. [Robert Calentine/Visuals Unlimited.]
b. Histone modification
• Histones are modified by acetylation of lysine, phosphorylation of serine and threonine and
methylation of lysine and arginine residues. (Fig 30-8, 17-2).
o Histones are the proteins where DNA is wrapped around as chromosomes and
chromatin
o Changes in histones then changes the binding of DNA to that protein
Acetylation of lysine or phosphorylation of serine reduces the overall positive charge of a protein.
• These are positively charged amino acid
• When it is modified via methylation/acetylation, it loses its positive charge and therefore
cannot bind to DNA
• When it is modifies by phosphorylation, the amino acid becomes negative, causing repulsion
of the DNA
• Both of these reduce the overall positive charge from proteins
Acetylation of H3 and H4 is associated with active chromatin, while methylation is associated with
inactive chromatin.
b. Histone modification
• Histones are modified by acetylation of lysine, phosphorylation of serine and threonine and
methylation of lysine and arginine residues. (Fig 30-8, 17-2).
• Generally the amino acids located at the N-terminal region of histones are targeted.
• Acetylation can occur
o at the time of DNA replication
o when genes need activation
• The acetylation of histone proteins alters chromatin structure and permits transcription
factors to bind to DNA.
• Deacetylated chromatin are more condensed structure. Following enzyme activities are
associated with acetylation/deacetylation of histones
o HAT- Histone acetyl transferase;
o HDAC- Histone deacetylase
o Two groups of HAT
• Group A- involved with transcription
• Group B –involved with nucleosome assembly
b. Histone modification
• It has been seen that methylation of cytosines in DNA is associated with condensation of
chromatin.
Document Summary
Regulation at genome level: chromatin decondensation, chromosomal puffs are regions of relaxed chromatin where active transcription is taking place. It is a visual evidence that chromatin decondensation is involved in genomic control. Chromosomal puffs on giant polytene chromosomes isolated from the salivary glands of larval. Following enzyme activities are associated with acetylation/deacetylation of histones: hat- histone acetyl transferase, hdac- histone deacetylase, two groups of hat, group a- involved with transcription, group b involved with nucleosome assembly, histone modification. It has been seen that methylation of cytosines in dna is associated with condensation of chromatin: bar bodies are extensively methylated x-chromosomes, regions of chromatin that has active genes are least methylated, azacytidine is an analogue of cytosine. It can be incorporated into dna but its structure does not allow its methylation. Incorporation of this analogue causes expression even from those genes that are normally silent.
