Part 3: Eukaryotic Regulation of transcription Flashcards
How do non-steroid hormones activate transcription of genes?
- by activating cell-surface receptors, which in turn activate intracellular signal transduction pathways
How do steroid hormones activate the transcription of genes?
- by directly stimulating gene transcription as hormone-receptor complexes.
Class II genes are:
- protein-encoding genes
- transcribed by RNA Pol II in mRNA
Chromatin:
- complex of DNA plus associated proteins
Steroid hormone receptors:
- soluble proteins
- act as transcriptional activators in response to hormone (steroid) binding
What other complex does a DNA-hormone-receptor complex interact with to stimulate the RNA pol II transcriptional machinery?
a “coactivator” complex
The two functional domains of all steroid hormone receptors:
- hormone binding domain
- DNA binding domain
- recognizes specific enhancer elements
Upon hormone binding, steroid receptors typically undergo a conformational change to create a:
- transcriptional activation domain
- binds a “coactivator” complex
Steroid response elements (SREs):
- DNA sequences that are enhancers for steroid responsive genes
- bind the hormone-receptor complexes, resulting in stimulation of gene expression
- typically palindromic, about 17bp
Agonists:
- bind hormone receptors
- stimulate receptor activity and gene expression
Antagonists:
- bind hormone receptors
- block receptor activity and represses gene expression
The three critical domains of all steroid hormone receptors (in general):
- ligand-binding domain
- activator domain
- DNA-binding domain
What does the DNA-binding domain of a steroid hormone receptor bind to?
- an enhancer element
- the steroid response elements (SREs)
Steroid response elements (SREs) are specific for:
- specific steroid hormone receptor - complexes
- SREs typically only differ by about 4bp
- therefore, mutations in SREs can bind different steroids, causing wrong gene expression
How far away are SREs from the promoter region of a gene?
- far, sometimes thousands of bps away from the promoter
Steps in the mechanism of transcriptional activation by steroid hormones:
- a hormone binds to the ligand-binding domain of a steroid hormone receptor
- the hormone-receptor complex binds to the SRE (an enhancer) via the DNA-binding domain of the receptor
- a transcriptional coactivator complex binds to the activation domain of the receptor that in turn is able to stimulation transcription
Two ways coactivators stimulate transcription:
- recruiting or stabilizing RNA pol II/GTF to the core promoter
- “clearing a path” for RNA pol II by altering the chromatin DNA template (chromatin remodeling)
Chromatin is made up of repeating units of:
nucleosomes
Each nucleosome contains:
- about 200 bp of DNA
- two copies each of histones H2A, H2B, H3 and H4 (“histone octamer”)
Steps by which steroid hormone receptors ultimately activate gene expression by altering chromatin structure:
- hormone binds to steroid hormone receptor
- a sequence-specific enhancer element (SRE) binds the hormone receptor-hormone complex
- a coactivator complex binds to displace or disrupt nucleosomes, thereby clearing a path for the binding of RNA polymerase II and its general transcription factors (TFIIA, TFIIB, etc.)
What proteins serve as chromatin remodeling proteins?
certain classes of coactivators
The two classes of chromatin-remodeling coactivators:
- catalyzes covalent modification of histone tails
- ATP-dependent remodeling complexes
- use the energy of ATP hydrolysis to remodel nucleosomes; do not covalently modify histones
What enzymes catalyze the acetylation of histone tails?
histone acetyltransferases (HATs)
(reversible with HDAC)
What enzymes catalyze the deacetylation of histone tails?
histone deacetylases (HDACs)
(reversible with HAT)
Most covalent modification during chromatin remodeling by coactivator complexes occur on what part of histones?
N-terminal tails
What types of covalent modifications can coactivator complexes make to histones during chromatin remodeling?
UMAP
- ubiquitination
- methylation
- acetylation
- phosphorylation
Mostly on N-terminal tails of the histones
Final result of chromatin remodeling:
- promoter region is taken out of the nucleosome loop and exposed so that RNA polymerase can bind to it
Five classes of steroid hormones:
PGAME
- PROGESTINS
- GLUCOCORTICOIDS
- ANDROGENS
- MINERALOCORTICOIDS
- ESTROGENS
- all cholesterol derivative
- each binds specific hormone receptors to activate different sets of genes
Anabolic steroids:
- agonists (trigger signalling pathways)
- bind to the androgen receptor to stimulate expression of genes involved in muscle development
Tamoxifen:
- antagonist
- acts as competitive inhibitor to estrogen estradiol
- binds to estrogen receptors, but does not stimulate signalling pathways (disables the receptor)
- fails to induce conformational change necessary for coactivator binding
Alpha-amanitin:
- toxin
- analagous to rifampicin
- Binds to RNA polymerase II in humans
- inhibits elongation stage of transcription
TFIIH:
- multi-subunit complex
- required for transcription initiation - unwinds so helices can start its job
- Mutations in two subunits of TFIIH cause XP, TTD, and CS - all a consequence of TC-NER inhibition
