3E- Regulation of Gene Expression Flashcards
What are operons?
in the bacterial genome, the structural genes for proteins involved in performing a related function are often grouped sequentially into units called operons
they are expressed together- are all either turned on or off
In bacteria, what is the principal means of regulating gene expression?
through repressors
they are regulatory proteins that prevent RNA pol to bind to the promotoe and thus act on the initiation of transcription
What are inducers?
They are small molecules that stimulate expressions of the operon by binding to the repressor and changing its conformation so that it can no longer bind to the operator
What are corepressors?
The repressor is inactive until a small molecule called a co-repressor binds to the repressor and activates it.
the repressor-co-repressor complex binds to the operator, preventing binding of hte RNA polymerase and gene transcription
Explain the process of the lac operon and outline the functions of the promoter, operator, repressor, and inducer in regulation of expression of the genes in that operon. Distinguish between the negative and positive controls in the system
If lactose is present and is needed to be digester, the cells adapt and produce 3 additional enzymes to digest the lactose, which are encoded by the lac opeeron.
Process:
a metabolite of lactose, allolactose, serves as an inducer –> binds to repressor and inactivates it –> RNA polymerase can bind to promotor and transcribe genes of lac operon
this only works if glucose is absent. if glucose is absent, cAMP increases –> CRP activated –> CRP binds to stimulatory region of operon.
If glucose is present, CRP doesnt work and txn is inhibited
What is the role of the sigma subunit in prokaryotic transcription?
sigma factors bind to the RNA polymerase and stimulate it to bind to a certain set of promotors, thus activating transcription of several operons
they are recruited in response to different stimuli, thus serving as a regulatory mechanism for the organism
What is transcriptional attenuation?
Use the trp operon to explain it
some operons are regulated by a process that interupps/attenuates transcription after it has been initiated
high levels of Trp attenuate transcription of the E. Coli trp operon
process:
at the 5’ end of the operon there are a ton of codons that require Trp-tRNA –> when Trp is high the cell, there is rapid translation of the transcript –> creates a hairpin loop and transcription terminates
When Trp levels are low –> Trp-tRNA levels are low –> ribosomes stall at codons for Trp –> different hairpin forms –> slower txn –> complete mRNA formed
Why can’t eukaryotic cells do attenuation?
in order for attenuation to work, you need stalling of the ribosome
since translation occurs outside of the nucleus, the ribosome cannot stall the transcription rate, and therefore the rate is independent on [substrate]
since prokaryotic cells simultaneously transcribe and translate, they can attenuate just fine
What occurs during chromatin remodeling in regulation of txn?
it refers to the displacement of nucleosome from specific DNA sequences so that txn of the genes in that sequence can be initiated.
2 mechanisms:
- ATP is hydrolyzed to unwind certain sections of DNA from the nucleosome core
- covalent modification of the histone tails through acetylation
What are histone acetyltransferases (HAT)?
they transfer an acetyl group from acetyl CoA to Lys residues in the tail
it removes a + charge from the amino group of Lys, removes the electrostatic interactions between histones and negatively charged DNA, and the DNA can unwind from the histones
What is the function of histone deacetylases (HDAC)?
they remove the acetyl groups from Lys resides, allowing the histones to wrap DNA tighter
What occurs during chromatin remoding in regulation of txn?
cytosine residues in DNA can be methylared to produce 5-methylcytosine
the methylated cytosines are locaed in CG rich regions called CG islands, which are near the promotor
genes that are methylated are less readily transcribed than those which are not methylated.
What is involved with the basal transcription complex?
Contains the TATA-bind protein (TBP) and other proteins called the basal transcription factors that form a complex with RNA polymerase II
What are gene-specfici transcription factors?
they bind to regulatory sequences and itneract with a mediator protein, such as a co-activator
by forming a loop in the DNA, co-activators interact with the basal transcription complex and can activate its assembly at the initiation site on the promoter
What are the 4 common structural motifs found in DNA binding proteins?
Give a little some some about em
- Zinc-Fingers: foudn in Dna-binding domain of steroid hormone receptors, Zinc chelated with His of Cys residues. Gives small, tight folded domain.
- Leucine Zippers: function as dimers to regulate gene transctiption. It’s an alph-heliz that contains Leu every 7 AA’s. 2 helices dimerize and “grip” the DNA through basic amino acid residues.
- Helix-turn-Helix: one helix fits into the major groove of DNA. It’s joined to a segment containing 2 additional helices that lie on top of one another.
- Helix-loop-helix: function as dimers that fit around and grip DNA similar to Leu-zipper proteins. The 2 helices are bound by a loop.
