Gene regulation

Gene expression models

Prokaryotes and Eukaryotes employ common and different methods of gene regulation

Prokaryotic models

1. Trp operon

2. Lac operon

Eukaryotic concept

1. Transcription factors

Anabolic tryptophan pathwayhttp://highered.mheducation.com/sites/0072995246/student_view0/chapter7/the_trp_operon.html

Tryptophan is an amino acid that must be made by E. coli through an anabolic pathway involving several enzymes

The trp operon encodes these enzymes

Operon - a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter.

Trp operon The trp operon consists of 5 genes

Trp operon Upstream of these genes is the promoter region (eg.

TATA box)

Within the promoter is a sequence called the operator

Trp operon A protein called repressor is able to bind the operator

when conditions are correct

In the absence of Trp Without tryptophan, the repressor protein cannot bind

the operator

In the absence of Trp With nothing in its way, RNA polymerase is free to

initiate transcription

In the absence of Trp With nothing in its way, RNA polymerase is free to

initiate transcription

This transcript will be translated into the enzymes capable of making tryptophan

In the absence of Trp With nothing in its way, RNA polymerase is free to

initiate transcription

This transcript will be translated and the cell will begin making Trp

In the presence of Trp When Trp is present it is able to bind to inactive

repressor protein

In the presence of Trp When Trp is present it is able to bind to inactive

repressor protein

In the presence of Trp The repressor is now able to bind to the operator

In the presence of Trp The repressor is now able to bind to the operator

This blocks RNA polymerase from transcribing the trp genes and the cell does not make Trp

Catabolic lactose pathwayhttp://highered.mheducation.com/sites/0072995246/student_view0/chapter7/the_lac_operon.html

Lactose is used by E. coli in the absence of glucose, its favourite carbohydrate food source

The enzymes responsible for breaking down lactose are under the influence of the lac operon

lac operon The lac operon is a bit more complicated and involves a

gene that is outside the operon

lac operon The lac operon itself contains 5 genes but control over it

is exerted by a regulatory gene called lacI

In the absence of lactose lacI encodes the lac repressor protein and is always

transcribed/translated into its active form

In the absence of lactose The active repressor protein blocks transcription of the

genes that encode enzymes for lactose metabolism

In the presence of lactose When E. coli has lactose available, -galactosidase

converts it into allolactose

In the presence of lactose When E. coli has lactose available, -galactosidase

converts it into allolactose

Allolactose binds the repressor protein and inactivates it

In the presence of lactose With the repressor out of the way, RNA polymerase is

free to do it’s job and transcribe the genes for lactose metabolising enzymes

In the presence of lactose With the repressor out of the way, RNA polymerase is

free to do it’s job and transcribe the genes for lactose metabolising enzymes

In the presence of lactose With the repressor out of the way, RNA polymerase is

free to do it’s job and transcribe the genes for lactose metabolising enzymes

http://highered.mheducation.com/sites/0072995246/student_view0/chapter7/the_lac_operon__induction.html

Eukaryotic gene expression

Eukaryotic gene regulation is similar to that in prokaryotes in that…

…protein-DNA interactions occur

But there are complicating factors such as…

…far more proteins are involved

…DNA is densely packed as chromatin

Transcription factors

Protein that bind to specific DNA sequences andcontrol their rate of transcription are called transcription factors eg. trp repressor and lac repressor

In eukaryotes transcription is naturally repressed by DNA packaging Transcription factors bind to promoter sites to keep DNA

available to RNA polymerase for transcription

Often numerous transcription factors are involved

Read pg. 312 – 313 to learn about the complexity of eukaryotic gene regulation