Lecture Notes

Regulation of bacterial gene expression- part 2
Learning outcome:

1. Control of the lac operon
2. Trp operon
3. Translational regulation

Overview of the regulation of the lac operon

 Comprise of 3 genes; lac Z,Y,A
 All 3 genes transcribed by RNA polymerase ll
 Sigma factor recruits RNA polymerase ll to the promoter
 RNA polymerase ll needs to be able to physically associate with promoter site
- Frequently RNA pol ll cannot because bacteria have overlapping operator and promoter sequences
- Repressor molecules bind to operators to prevent RNA polymerase ll binding to promotor
- No transcription
 The lac operon is permanently silenced in bacteria
- No need to waste energy synthesising lac operon in the absence of lactose (disaccharide)
 In the presence of lactose (substrate) the lac operon needs to switch production of genes on
 The lac repressor, operator and promoter need to be sensitive to the presence of lactose
 The repressor has the ability to bind to allolactose (isomer of lactose)= repressor loses affinity to
operator sequence = repressor falls off.
 Sigma factor can recruit RNA polymerase ll to promoter sequence and transcription occurs

Lac repressor

 The lac repressor is encoded by a regulatory gene
- Regulatory gene = regulatory protein =
function to regulate something else.
 Distinct from the lac operon
 has own promoter
 therefore, when lac operon is repressed, the lac
repressor synthesis still occurs




The lac operon has a second layer of control

 what happen if grow bacteria in a culture of glucose and lactose
 Black line: growth of bacteria in culture

, - There are 2 distinct growth phases (gradual increase in absorbance and time)
- Lag phase in between the 2 cycles of e.coli growth
 Red line: lac operon is transcribed after 50 minutes
 Why is bacteria switching lac operon gene after 50 minutes and not from the start?
- Result: E.coli uses glucose in preference to lactose
- Use lactose once glucose has been used up
 This lead to the proposal that glucose must be repressing transcription of the lac operon
 Why does e.coli prefer glucose over lactose
- Glucose is more energetically favourable to metabolise as goes straight in to the TCA cycle
- Lactose has to be broken down into glucose and galactose
- Galactose has to metabolised ultimately to feed into the TCA cycle




How does glucose prevent the expression of the lac operon?

 Called carbon catabolite repression (or just carbolite repression)
 Synthesis of catabolite repressor protein
 Catabolite repression of lac operon through glucose is mediated by another transcriptional regulator called
CRP
 CRP is required for the activation of the lac operon
 CRP transcribed from own regulatory gene in an inactive form
 CRP is an activation molecule
 CRP can only activate transcription when itself is activated
 CRP is activated by a small molecule, called cAMP ( cyclic adenosine monophosphate)
 cAMP is a second messenger (small-molecule inducer)
 cAMP is a variation of ATP, ADP, AMP family
 cAMP is a ubiquitous (present in all types of cells; eukaryotes and prokaryotes) signalling molecule
 cAMP binds to the inactive CRP, which changes its conformation (cAMP-CRP)= active CRP that directly
interacts with RNA polymerase ll and transcription occurs.
 POSITIVE INDUCIBLE CONTROL SYSTEM