Control of Gene Expression Flashcards
Parts of the lac operon
Regulatory Gene: binds repressor
Promoter- has the operator and CRP gene
z gene: beta galactosidase –> lactose –> glucose + galactose
y gene: galactoside permease –> lactose –> into cell
a gene: thiogalactoside transacetylase –> ?
^genes right next to eachother: polycistronic message
lac operon- glucose only
1) regulatory gene creates mRNA for repressor protein
2) repressor binds to operator
3) RNA polymerase cannot bind promoter
4) transcription blocked
lac operon- little lactose present
what does beta galactosidase make?
beta galactosidase
lactose –> glucose + galactose
OR
lactose –> allolactose (isomer)
What happens when little lactose present in the lac operon?
1) regulatory gene creates mRNA for repressor protein
2) allolactose binds to repressor
3) repressor cannot bind operator
4) RNA polymerase binds to promoter
5) transcription…but at a LOW level
How are cAMP and glucose levels related?
increase glucose, decrease cAMP
decrease glucose, increase cAMP
lac operon: lots of lactose and glucose
1) high CRP levels –> but doesn’t do anything
2) RNA pol binds to promoter
3) low transcription
lac operon: lots of lactose and little glucose
1) CRP and cAMP bind leading to a conformational change in CRP
2) bind onto the promoter
3) Stimulates binding of RNA polymerase to the promoter
4) Transcription level is high
Helix-turn-Helix motif (transcription motif)
Recognition binds major groove
dimer alpha helices are bound to the major groove of DNA
H bonding between bases on DNA and the protein end up becoming really strong since there are so many bonds made –> this makes this process very specific
ie: CRP (cAMP receptor protein)
Function of CRP
acts as a dimer on the major groove to give a wide array of protein possibilities for interactions
Pitt Hopkins Syndrome
Creates mutation in the basic region of a transcription factor…affects DNA binding of the transcription factor
Arg 576 highly conserved and essential for DNA recognition
Zn (transcription motif)
alpha helices bind major groove
not a dimer
Use of 1 or more molecules of Zn
Repetitive motif of 2 cystine and 2 histidine (or cystine) residues in the sequence
Repetitive so can maker different fingers
Also has specific and strong interactions with DNA
Leucine Zipper/bZip (transcription motif)
dimer alpha helices are bound to the major groove of DNA
Has 2 parts:
1) Zipper alpha helix: coil formed so that leucines stick out on one side –> hydrophobic interactions
2)Basic region DNA binding alpha helix: this is the part that interacts with DNA
Basic helix-loop-helix (transcription motif)
dimer alpha helices are bound to the major groove of DNA
has alpha helix non helical loop alpha helix on the top
Bottom is a basic region- DNA binding and an alpha-helix
How do you make different zipper motifs?
Different amino acids can replace the leucines on the zipper alpha helix
ie: arginine, histidine
Ways to regulate transcription factors
1- protein synthesis 2- ligand binding 3- protein phosphorylation 4- Addition of subunit 5- unmasking 6- Stimulation of nuclear entry 7- release from membrane
How do estrogen receptors work?
Estrogen receptor and ligand estrogen bind to estrogen response element (ERE)
this binding unmasks the receptor and allows for transfer into the nucleus –> bind to coactivaters in nucleus –> transcription
DNA methylation and cancer
DNA methylation suppresses the expression of tumor suppressor genes in cancer
How does methylation block gene expression?
1) Methylated CpG islands inhibits transcription by physically precluding transcription factor recruitment
2) Methylated CpG binding protein 2 (MeCP2) binds to methyl groups and prevents transcription by recruiting chromatin recruiting factors
Rett Syndrome
mutation in MeCP2 gene –> over transcription
Methylation can also lead to gene activation
ie: Igf2
Insulin-like growth factor- imprinted gene
Why in Igf2 are the paternal genes expressed even if they are methylated?
Methylation is at a DMR (differentiated methylated region) and promoter is open. Methylation at this regulatory region of the paternal chromosome prevents binding of a repressor to the region and transcription can occur
Cutaneous T-cell lymphoma
Treated with HDAC inhibitor so that tumor surpressor gene expression increases
HDAC inhibitor –> more acetylation of histones
Nucleosomes and gene expression
Positive superhelical tension
ATP-driven chromatin remodeling
-remodeling factors interact with C terminal of large subunit of RNA polymerase II and use ATP to open up nucleosome
Covalent Modification of histone tails
-Methylation of lysine or arginine (HMT
3 ways of modifying histone tails
1) Acetylation of lysines
2) Phosphorylation of serines
3) Methylation of lysines and arginines
Acetylation of histone tails
acetylation thru HAT reduces amount of + charge –> becomes loosely associated with DNA backbone –> can do transcription!
HDAC deacetylates and pushes reaction opposite way
Thyroid Hormone Receptor- HAT and HDAC
Thyroid receptor and Reitinoid X receptor when attached to DNA both are attached to one another –> attratcs a co-repressor –> deacetylates histones –> tightly wrapped DNA –> transcription not initiated
Thyroid Hormone (T3) changes conformation of TR/RXR –> corepressor doesn’t bind –> coactivator with HAT can attach –> transcription!
Methylation on lysine
can activate and repress!
