CSB349H1 Lecture Notes - Lecture 4: Repressor, Methyltransferase, Histone Deacetylase
14 May 2018
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Lecture 4(c): Transcription II
Transcriptional Repression:
• Transcriptional repression can be mediated similarly to the way of transcriptional activation
o Activation can only occur via one way: must recruit transcriptional machinery
1. Competition:
o Repressor can inhibit binding of an activator to a gene
§ Due to overlapping DNA sequences between activator and repressor-
therefore, are competing for the same binding sport
2. Inhibition:
o Repressor has a distinct binding site close to the activator; once the repressor binds, it
will interact with the activator and prevent the activator from interacting with
other components
3. Direct Repression:
o Repressor binds to basal transcriptional machinery directly
§ Independently of the activator
4. Indirect Repression:
o Repressor recruits chromatin modifying factors
§ HDAC: spread repressive chromatin marks all over the region of the
chromatin and cover the start of the gene turning it off
• Also capable of turning several gene transcription off
Polycomb Repressor Complexes (PRCs):
• Large multi-subunit protein complexes- a form of long-range repressor
• Repressing transcription factors have a domain that recruits PRC
o PRC has a subunit that contains histone methylase transfer activity and beings
methylating histones
o PRC has a subunit that contains chromodomains which binds to the methylated
histones
ð Leads to a methylated and bound state of histones; keeping the chromatin in an extremely
repressed/silenced state
o PRC includes both Class I and Class II enzymes
1&2: short range
3&4: short/long range
