Lecture 18 Eukaryotic Transcription 2 Flashcards
Chromatin remodelling
Determines which regulatory regions are accessible adding another level of regulation
Acetylated chromatin - acetylation of the histones - open and transcriptionally active (RNA pol can get in)
De acetylated chromatin - closed and transcriptionally repressed
Example of deacetylation : flowering FLC and FLD
1) acetyl groups on histone proteins destabilise chromatin structure
2) flowering locus C (FLC) encodes a regulator protein that represses flowering
3) no flowering takes place
4) flowering locus D (FLD) encodes a deacetylase enzyme
5) this removes acetyl groups and restores normal chromatin structure
6) no transcription of FLC takes place
7) flowering is not suppressed - flowering takes place
Post transcriptional control
Small RNA molecules can interfere at 2 levels
Regulatory systems that all use dsRNA
Discovered 1998 attempting antisense in nemotodes, found dsRNA more effective for knocking out functions
Virus defence and gene regulation - detected by host and virus genes silenced
Small RNAs (~22nt)
Gene silencing
Small RNA’s in regulation: non coding RNA
Non coding RNA
- involve formation of dsRNA
-chopped into pieces by DICER enzyme
-pieces later used for regulation
-pieces are short dsRNA (~22nt) with regulatory functions
Small RNA’s in regulation: micro RNAs or miRNA
RNA transcribed from special micro RNA genes that never goes onto make a protein - makes miRNA
Origins:
Some cleaved from RNA transcribed by special miRNA genes
Others encoded in the introns and exons of mRNA (transcripts)
Cleaved from a single stranded RNA precursor that form small hair pins
Small RNA’s in regulation: small interfering RNA (siRNA)
Origins: from cleavage of
mRNA
RNA transposons
Or RNA viruses
(used by host to switch off vital genes)
Multiple siRNAs are produced from the cleavage of an RNA duplex consisting of two diff RNA molecules coming together
miRNAs regulate translation
1) transcription through an inverted repeat in DNA
2) produces a primary miRNA (pri-miRNA)
3) the pri mRNA is cleaved to produce a short RNA with a hairpin
4) DICER removes terminal loop of hairpin
5) one strand of the miRNA combines with proteins to form an RNA-induced silencing complex (RISC)
6) which pairs with an miRNA and inhibits translation
7) inhibition of translation
siRNAs cause mRNA degredation
1) dsRNA may arise from RNA viruses or very long hairpins
2) cleaved by DICER into a lot of short silencing dsRNA pieces
3) producing siRNAs
4) an siRNA combines with proteins to form RISC
5) which pairs with and cleaves mRNA leading to degradation
6) cleavage and degredation
Comparing siRNA and miRNA
miRNA inhibits translation siRNA inhibits transcription
Origin:
siRNA- mRNA, transposon or virus
miRNA - RNA transcribed from a distinct
gene
Cleavage of:
siRNA - RNA duplex or ssRNA that forms long hairpins
miRNA - ssRNA that forms short hairpins of dsRNA
Size
siRNA and miRNA - 21-25 nucleotides
Action
siRNA - degradation of mRNA, inhibition of transcription, chromatin modification
miRNA - degradation of mRNA, inhibition of translation, chromatin modification
Target
siRNA - genes from which they were transcribed
miRNA - genes other than the ones from which they were transcribed
More on miRNA
MicroRNA interaction with regions including the 5’ UTR, coding sequence and gene promotors can occur (as well as 3’ UTR)
Can be secreted into extracellular fluid in animals and transported to target cells via vesicles such as exosomes or by binding proteins
Extracellular miRNAs function as chemical messengers to meditate cell-cell communication
De- regulation of miRNA controlled pathways can lead to disease particularly forms of cancer
