gen bio - lecture 16: gene regulation

constitutive genes

always needed + constantly transcribed

when does prokaryotic gene regulation occur

@ transcription level (DNA → RNA)

escherichia coli

• normal gut bacteria in human intestine

• can adjust based on host diet

• humans drink milk… E. coli digests lactose

induction

turning ON gene expression

inducer

compound that stimulates gene expression

inducible gene

gene activated by inducer

• default state is OFF - turned on when needed 

operon

a DNA unit that coordinates the expression of multiple related genes

it includes…

• structural genes (shared function)

• regulatory sequence (control expression)

lac operon model components (3)

1. promoter: RNA polymerase binding site - NOT TRANSCRIBED

2. operator: on/off switch sequence for gene expression

3. 3 genes: lacZ, lacY, lacA

negative regulation (def)

repressor prevents transcription

positive regulation (def)

transcribed…but only a little

negative regulation is determined by…

the presence or absence of lactose

• no lactose = operon OFF, no expression

• yes lactose = operon induced

lac repressor activity

• no lactose = repressor binds to lac operator

• RNA polymerase can bind proper BUT CANNOT TRANSCRIBE GENES

no lac operon expression!

lac induction via allolactose

• lactose enters cell + is converted to allolactose (is the inducer)

• binds the second site on repressor protein → inactivates repressor

• allows transcription to occur

low levels of lac expression

^^ what is transcribed (lac induction via allolactose)

• mRNA w/all three genes

  • each gene has it’s own start + stop codon

  • translated as 3 separate polypeptides

• no nucleus —- transcription and translation occur simultaneously

positive regulation responds to the presence or absence of…

glucose!!!

lac promoter + CAP

• lac promoter alone is inefficient! and has low affinity for RNA polymerase

• solution? CAP

  • increases affinity of promoter for RNA polymerase

  • CAP is inactive alone!! only active when bound with cAMP

how does CAP-cAMP work

• low glucose = cAMP increases

• CAP-cAMP binds CAP-binding site next to RNA polymerase binding site

  • bends double helix

  • increases RNA polymerase binding

result: high levels of lac expression

eukaryotic gene regulation regulates…

EVERY LEVEL

regulation of chromatin structure

• interphase

• if DNA is loosely packed, genes = active

  • if densely packed = NO GENE EXPRESSION

modified via DNA METHYLATION

regulation of transcription initiation

• involves DNA sequences + transcription factors

  • promote or inhibit transcription initiation

• alternative splicing: different polypeptides used from the same gene

post transcriptional regulation

• length of poly-a tail affects levels of translation!!

  • enzymes degrade tail = makes it shorter and shorter

post translational regulation

• polypeptides are often processed BEFORE forming final protein (chemical mods)

• must be folded correctly!!