Regulation of Gene Expression

1
Regulation of Gene Expression

• Prokaryotes and Eukaryotes

2
Regulation of Gene Expression

• A cell contains the entire genome of an organism
  ALL the DNA.
• Gene expression transcribing and translating
  the gene
• Regulation allows an organism to selectively
  transcribe (and then translate) only the genes it
  needs to.
• Genes expressed depend on
• the type of cell
• the particular needs of the cell at that time.

3
Gene Regulation in Prokaryotes

• Prokaryotes organize their genome into operons
• Operon a group of related genes
• One promoter sequence at the very beginning
• All of the genes will be transcribed together (in
  one long strand of RNA.

4
Question

• What is the benefit of organizing the genome into
  operons?
• Its more efficient transcribe everything you
  need for a process at once.

5
Repressible Operon Trp Operon

• Repressible Operon Operon that is usually ON
  but can be inhibited
• The Trp Operon
• example of a repressible operon
• Genes that code for enzymes needed to make the
  amino acid tryptophan

6
TrpR Gene

• TrpR gene is the regulatory gene for the Trp
  operon
• Found somewhere else on the genome
• NOT part of the Trp operon
• TrpR gene codes for a protein TrpR repressor
• TrpR gene is transcribed and translated
  separately from the Trp operon genes.

7
TrpR Repressor

• Repressor protein is translated in an inactive
  form
• Tryptophan is called a corepressor
• When tryptophan binds to the TrpR repressor, it
  changes it into the active form

8
Operator Region

• There is also an operator region of DNA in the
  Trp Operon
• Just after the promoter region
• The TrpR Repressor can bind to the operator if
  its in the active form

9
Trp Operon

• Transcription is ON
• Occurs when there is no tryptophan available to
  the cell.
• Repressor is in inactive form (due to the absence
  of tryptophan)
• RNA Polymerase is able to bind to promoter and
  transcribe the genes.

10
Trp Operon

• Transcription is OFF
• Occurs when tryptophan is available
• Tryptophan binds to the TrpR repressor ? converts
  it to active form
• TrpR protein binds to operator ? blocks RNA
  Polymerase ? no transcription

11
Question

• Under what conditions would you expect the trp
  operon to go from OFF to ON again?
• When there is no longer tryptophan available all
  of it has been used up

12
Inducible Operon Lac Operon

• Inducible operon operon is usually OFF but
  can be stimulated/activated
• Lac Operon
• Example of an inducible operon
• Genes code for enzymes that break down lactose

13
LacI gene

• LacI gene is the regulatory gene for the lac
  operon
• Found somewhere else on the genome
• NOT part of the lac operon
• LacI gene codes for a protein lacI repressor
• LacI gene is transcribed and translated
  separately from the lac operon genes.

14
LacI Repressor

• The lacI repressor protein is translated into an
  active form
• When the lacI repressor is bound by lactose (also
  called allolactose) it becomes inactive
• Lactose is the inducer

15
Lac Operon

• Transcription is OFF
• When there is no lactose that needs to be
  digested
• lacI repressor is in active form ? binds to
  operator ? blocks RNA Polymerase ? no
  transcription

16
Lac Operon

• Transcription is ON
• When there is lactose that needs to be digested
• Lactose binds to lacI repressor ? inactivates it
• RNA Polymerase is able to bind to promoter ?
  transcribe genes

17
Do all operons have operator regions?

• NO
• There are some genes that always need to be
  transcribed ? they do not need to have operators
  to regulate them in this manner.
• Ex. genes that participate in cellular respiration

18
Positive Gene Regulation

• In the lac operon there are other molecules to
  further stimulate transcription.
• Lactose will only be digested for energy when
  there isnt much glucose around
• When glucose levels are low, level of cAMP
  molecule builds up

19
cAMP and CAP

• CAP regulatory protein that binds to cAMP
• CAP is inactive unless cAMP binds to it

20
Positive gene regulation

• If there isnt much glucose? high levels of cAMP
• CAP and cAMP bind ? CAP can bind to the promoter
  ? stimulates RNA Polymerase to bind

21
Positive gene regulation

• When glucose levels rise again, cAMP levels will
  drop ? no longer bound to CAP
• CAP cant bind to promoter ? transcription slows
  down

22
Positive gene regulation

• The lac operon is controlled on 2 levels
• Presence of lactose determines if transcription
  can occur
• CAP in the active form determines how fast
  transcription occurs

23
Gene Regulation in Eukaryotes

• Eukaryotes have large genomes
• Other molecules have to help RNA Polymerase find
  the promoter and start transcription
• Transcription factors
• Enhancer sequences

24
Transcription Factors

• Series of proteins that bind to the promoter to
  help RNA Polymerase bind
• RNA Polymerase also has to bind transcription
  factors in order to be able to start
  transcription.

25
Question

• How might binding transcription factors help RNA
  Polymerase bind?
• Creates an area that chemically attracts RNA
  Polymerase more

26
Enhancer sequences

• Sequences of DNA that are far away from the gene
  they help transcribe
• Process
• Activator molecules bind to the Enhancer sequence
• Enhancer loops around so that the activators can
  also bind to the transcription factors
• Together with RNA polymerase they all cause
  transcription to start

27
Cell-specific Regulation

• Each cell has the DNA to transcribe any gene
• Different activators and transcription factors in
  specific cells will determine which genes are
  transcribed ? which proteins are translated

28
(No Transcript)