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 Upstream of these genes is the promoter region (eg.
TATA box)
Within the promoter is a sequence called the operator
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 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 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