Regulation of Gene ExpressionRegulation of Gene Expression

Prokaryotes and Eukaryotes

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.

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.

Question…

• What is the benefit of organizing the genome into operons?– It’s more efficient – transcribe everything you

need for a process at once.

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

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.

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

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 it’s in

the active form

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.

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

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

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

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.

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

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

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

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

Positive Gene Regulation

• In the lac operon there are other molecules to further stimulate transcription.

• Lactose will only be digested for energy when there isn’t much glucose around

• When glucose levels are low, level of cAMP molecule builds up

cAMP and CAP

• CAP = regulatory protein that binds to cAMP

• CAP is inactive unless cAMP binds to it

Positive gene regulation

• If there isn’t much glucose high levels of cAMP

• CAP and cAMP bind CAP can bind to the promoter stimulates RNA Polymerase to bind

Positive gene regulation

• When glucose levels rise again, cAMP levels will drop no longer bound to CAP

• CAP can’t bind to promoter transcription slows down

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

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

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.

Question…

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

Polymerase more

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

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