CSB349H1 Lecture Notes - Lecture 3: H2Afz, Histone Code, Transcription Preinitiation Complex
14 May 2018
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Lecture 3(d): Transcription
Nucleosome Remodeling:
ð Below are a few mechanisms of how class II enzymes function on the nucleosomes:
a) Remodeling:
o Change in the nucleosome structure but no change
in position
b) Sliding:
o Displaces nucleosome along the DNA
c) Transfer:
o Removing and transferring nucleosomes to non-adjacent region of DNA
Nucleosomes and Transcription in Yeast:
Ways to measure relatively density of nucleosomes:
a) DNase I Hypersensitive Assay:
o Promoters immediately found upstream of actively transcribed genes are found in
regions of relatively uncondensed chromatin – high hypersensitivity
§ Sites are referred to as DNase hypersensitive sites (DHS)
• Open sites of hypersensitive chromatin are associated with the histone variant H2A.Z
(instead of H2A) – alternative histones included in the nucleosome
ð Presence of DHS upstream of a gene correlate with whether or not a gene is actively
transcribed or not
o Nucleosome-free region (hypersensitive region) is associated with active transcription
Experiment– Drosophila hsp70
• Hsp70 gene is normally turned off which respond when the
drosophila is treated with high temperature
o Heat-shocking the cell allows us to visualize the formation of DHS
sites upstream of the gene; activating transcription via HAT complex
Model of Nucleosome Remolding in Yeast:
• We begin with a condensed form of chromatin – DNA is wrapped around the nucleosomes
1. Transcription factor binds onto a promoter region and being recruiting
factors that function to decondense chromatin
o SAGA complex (Class I) is recruited which functions
to acetylate histones, as well as H2A.Z containing
nucleosomes
2. Acetylated histones are recognized by Class II enzymes and
are recruited to further remodel the chromatin
o SWI/SNF complex (bromo-domain) functions to
loosen up the chromatin
