Lecture 11 Flashcards
(18 cards)
What are negative regulators?
Repressor proteins that bind to DNA sequences and prevent transcription
What are positive regulators?
Activator proteins that bind to DNA sequences and initiate gene expression
How many polypeptides does the lac operon produce?
3
What is an operon?
A cluster of genes that are involved in biosynthetic processes and expressed from a single promoter
What does the regulatory region in the lac operon have?
The promoter region
The operator sequence to bind the lac repressor protein
Cap binding site to bind CAP-cAMP
The protein coding genes: Lac-Z (codes beta-galactosidase), Lac-Y (encodes lac permease), Lac-A (encodes transacetylase)
What does Lac-I gene code for?
The lac operon repressor protein (housekeeping gene)
What is the lac repressor?
A protein that is a homotetramer which has a domain that binds to the operator region and a domain that binds to the inducer (allolactose)
What does beta-galactosidase do?
Breaks down lactose into galactose and glucose and produces allolactose
What does allolactose do?
Binds to the lac repressor protein and causes it to dissociate from the operator sequence which allows transcription to occur
What happens in bacteria when no glucose is present?
cAMP is produced which binds to CAP to form CAP-cAMP
CAP-cAMP binds to CAP binding site and promotes transcription of lac operon
What happens if lac-I has a mutation?
The mutant lac repressor will never bind and lac operon will always be expressed
What happens if lacO has a mutation?
Operator sequence is mutated and lac repressor protein cannot bind to it
What is arabinose?
A pentose sugar that comes from plant cell walls
When is the arabinose operon expressed?
When arabinose is present and glucose isn’t
What enzymes are inside the arabinose operon?
araB, araA and araD
They breakdown arabinose into an intermediate to be used in glycolysis
What is araC?
A singular regulatory protein which carries out positive and negative regulation
It has two domains, one binds to DNA and another to arabinose
What happens when no arabinose is present?
Two araC bind to AraI and AraO2 and then bind to each other, creating a tight DNA loop to prevent RNA polymerase binding to promoter and cAMP-CAP complex binding to CAP binding site
What happens if arabinose is present?
It binds to arabinose binding site of two AraC monomers which breaks the loop
Allows room for RNA polymerase to bind to the promoter region
Additional AraC bind to other binding sites which increases chance of transcription
cAMP-CAP binding to CAP binding site which allows RNA polymerase to bind more efficiently to Para promoter to initiate transcription
