Gene expression -MCB Prokaryotes

What is central dogma

Genome

Transcriptome (RNA)

Proteome (PROTEIN)

RNA Polymerase

Interact with DNA

Initiate transcription

P -ve regulation

repressor binds operator blocks RNA polymerase

P +ve regulation

activator enables transcription

(somes need activator RNA polymerase to bind)

what do inducers do

bind repressors can't bind - conformational change

E.coli energy source

Glucose - source carbon + energy

Absence glucose use lactose

How does E.coli get glucose from lactose

β- galactosidase hydrolyse

lactose → galactose + glucose

what 3 enzymes response to specific change environment

β- galactosidase

Galactose permease = transport lactose cell membrane

Thiogalactoside transacetylase

(acetylation + detoxification transported by permease)

Operon

Has promoter, operator

5' to 3'

produce polystronic mRNA (Multiple)

lac operon structure

3 genes → lacZ, lacY, lacA

LacI - repressor gene

What does lac operon do lack lactose

repressor binds operator - prevents transcription

lac operon when there is lactose

lactose bind lacI - conformational change

Callosteric regulation - repressor can't bind operator

E.coli glucose better carbon source than lactose

Lactose version

Lactose: Lac operon not completely repressed by inhibitor

Lactose permease enzyme transport little lactose in cell

Low level transcription

E.coli glucose better carbon source than lactose

Glucose version

Glucose transported in cell - phosphotransferase system

Cause inactivation lactose permease enzyme

Carbon Catabolite Repressor [CCR]

(applies to other operons not lac operon)

Glucose lowers the concentration cAMP

Catabolite Activator protein (CAP) and cAMP bind - region upstream start transcription site

CAP + cAMP complex - cause RNA polymerase to bind

CCR activator

activator stimulates transcription

absence CAP cAMP complex low expression

Tryptophan (Trp) Operon

Gene encode 5 enzymes convert chorismate → tryptophan

Operon expressed Trp levels low

Trp level high - repressor switches off expression

Trp repressor

Trp repressor bind operator when also bound tryptophan

Conformational change allow bind DNA

(opposite lac repressor)

Regulation Trp

RNA polymerase binds promoter initiates transcription - operon on

Tryptophan binds to active repressor, conformational change allows to binds to DNA - operon off