CBG Lecture 5: Regulation of transcription Flashcards
what is transcription mainly controlled by
cis-acting DNA elements
trans-acting DNA elements`
how can gene expression in prokaryotes be modified
by an interplay between transcription and translation
what is attenuation
an interplay between transcription and translation
what are cis-acting DNA elements?
regulate the DNA close to them by binding gene regulatory proteins
eg. promoter/operator/enhancer
sequences that have to be physically next to eachother to work are cis
give some examples of cis acting DNA elements
promoter
operators
enhancers
what are trans-acting DNA elements?
must be expressed a gene-regulatory protein to act ad can be relatively far away, even on different chromosomes eg. regulator DNA sequence
examples of trans-acting DNA sequence: Transcription factors, repressors, inducers
give some examples of trans-acting DNA elements?
transcription factors
repressors
inducers
what are transcription factors
DNA binding roteins
give some examples of the main types of transcription factors and their structures
TATA binding protein (TBP) - beta scaffold (unusual for TF)
activator protein 1 (AP1) leucine zipper - generally binds in the major groove
CAP -catabolite activator protein (helix-turn-helix)
Early growth response protein 1 (EGR-1) -> Zinc fingers
what is a TBP -give main structure
TATA binding protein
beta scaffold
what is an AP-1 -give main structure
activator protein transcription factor
leucine zipper-generally binds in the major groove and straddles the DNA
what is CAP-give main strucutre
transcription factor: catabolite activator protein: helix-turn-helix
what is EGR-1-give main strucutre
early growth response pritein 1 - zinc fingers - have histidine/cysteine residues. fingers poke into major groove
why does there need to be transcriptional regulation
so only express the proteins that are needed, as all cells have the same genome, but different genes expressed in different cells
different condns of same cell = different proteins needed
what names do DNA sequences bound by TFs go by
low level sequences
OR
high level assemblies
what are low level sequences?
response element = DNA sequence recognised by protein
eg. in bac: Pribnow box (E.coli) in euk: TATA,CAAT,BRE box ->assembled with large units
what are high level assemblies?
DNA sequences bound by promoter =RNAP binding
for bac: operator
euk: enhancer, silencer, insulator
what 2 things do trans regulatory elements need, to work
through interaction between
1)a transcribed and translated TF derived from the trans-regulatory element AND a DNA regulatory element thats adjacent to the regulated gene
what control is the trp operon under
negative repressible control
what is the trp operon regulated by
repressor protein
attenuation
what is the trp operon composed of
trpR gene which synthesizes repressor protein
P:Promoter region where RNAP binds
O-operato region where effector binds (either a repressor or inducer) then structural gene regions trpA to L)
the structural genes encode the enzymes for trp synthesis
what does repressible control mean
transcription is ON unless an effector is added
what does negative control (of an operon) mean
transcription is turned OFF by binding to regulatory protein
what are operons
they are how orikaryotes regulate gene sequences
what happens to a prokaryote when concentration of tryptophan is high
- repressor binds to co-repressor (trp) and becomes able to bind DNA
- repressor/corepressor complex blocks transcription by binding operator
- no tryptophan structural genes transcribed
what happens to a prokaryote when concentration of tryptophan is low
turns the trp operon ON
not enough typ available for TrpR to be in DNA-binding form
No TrpR/trp blocking the operator so RNAP can transcribe operon
what is autologous repression
give an example of this
repression by an element
transcribed within the regulated gene
in the lac operon, lacI represses its own transcription
besides repressors, how else is the trp operon controlled
by attenuation
why does attenuation not happen in eukaryotes
because you need interplay between transcription and translation without compartmentalisation using membranes (membranes present in EUKS)
overall, what does attenuation do to trp operon
can prevent expression of trp biosynthesis genes when trp levels in cell are high
how does attenuation work at high tRNAtrp concentration
ribosome reads through 1 easily and shields 2, 3&4 form a transcription terminator hairpin
how does attenuation work at low tRNAtrp concentration
ribosome stalls in 1 waiting for tRNAtrp;
2 binds 3
3&4 cant form transcription terminator hairpin
what happens when a 3&4 stem loop is present in trpL mRNa
when a 2&3?
it acts with the attenuator (uracil rich sequence) to form a terminator for transcription at end of trpL gene
when a 2&3 stem loop forms, attenuator doesnt form and transcription proceeds to trpE
what determines whether ribosomal stalling will happen in mRNA
two adjacent trp codons being present in ribosome
what are eukaryotic genes controlled by?
give flow chart of control
numerous enhancers
enhancer -> mediator -> basal TFs + RNAP (from a long looped distance)
what sequence does the lac operon work by
tran acting sequence
what genes are involved in the lac operon
lacI - codes repressor protein
CBS - CAP binding site (catabolite activator protein binding site)
Promoter
Operator
structural genes: lacZ - beta galactosidase
lac Y - lactose permease
lac A - importer protein
how does the lac operon work
under negative inducible control
repressor is inactivated by binding inducer - lactose
repressor blocks transcription by binding operator
what happens when E.coli has high concentrations of glucose and low [lactose]
lacI transcribed and translated
repressor blocks transcription by binding operator
RNAP cant bind therefore no structural genes transcribed
what happens when E.coli has low [glc] and high [lac]
lacI transcribed and translated
repressor blocking transcription by binding operator is INACTIVATED by binding allolactose inducer
no lacI blocking operator so RNAP can transcribe operon
what two ways is lac operon controlledq
positive control by CAP
negative inducible
how is lac operon under positive control
by CAP
low [glc] -> high [cAMP] -> CAP/cAMP able to bind CBS(CAP binding site)
transcription is turned ON when regulatory protein (activator is CAP) binds to DNA and actively recruits RNAP
