Lecture 24: Gene Control In Prokaryotes Flashcards
Describe the difference between positive and negative gene regulation in prokaryotes
Negative control: the presence of a repressor protein prevents transcription ie the repressor turns a gene off
Positive control: the presence of an activator protein initiates transcription ie the activator turns a gene on
How is lactose metabolised in prokaryotes?
When lactose is present in a system a catabolic enzyme B-galactosidase is produced to cause the hydrolysis of the disaccharide into monosaccharides.
The synthesis of this enzyme is dependant in substrate induction.
There are 3 main genes involved in lactose metabolism, Lac Z, Y and A. All three belong to a unit called an operon.
Transcription of these genes is dependant in the presence if the inducer, which in this case is lactose.
What is an operon?
Group of genes with related functions that are clustered together with DNA sequences that allow the genes to be turned on or off simultaneously”.
Give the definition of an inducible operon
And
Repressible operon
Inducible operon- an operon that is transcribed only in the presence if a specific small molecule inducer eg lactose
Repressible operon- an operon that is transcribed only in the absence of a small molecule co-repressor
Give the definition of constitutive production
Enzyme is produced all the time by the cell, regardless of the presence or absence of inducers or repressors
The operon also contains 2 control sites. What are they and give a definition of each
Operator (O): binds the repressor (the production of the lacI gene). So its the site of repressor binding.
Promoter (P): RNA polymerase binding site
Describe the regulation of the lac operon in E.coli
A regulatory gene works in close contact with the operon. The regulatory gene continuously produces the repressor protein LacI.
In the absence of lactose the regulatory protein binds to the operator site, blocking RNA polymerase, thus the proteins required for lactose metabolism aren’t produced.
If lactose is present, then the repressor molecule binds with the inducer (lactose), changing its shape so that it can’t bind to operator site, allowing RNA polymerase to bind and the proteins for lactose metabolism are produced.
What is catabolite repression?
In the presence if glucosamine levels of cAMP are reduced as glucose indirectly inhibits an enzyme which produces cAMP. The more glucose in a cell the less cAMP is made. As a consequence, as there is no cAMP to bind to CRP, there is no complex to bind to the promoter and thus no transcription occurs
What is a constitutive mutant?
This results from a mutation in either the lacI gene or the operator ie
LacI gene mutation: non-functional repressor is produced which can’t bind to the O site
Operator mutation: changes in O site such that repressor can no longer bind.
Either way there is no bonding of the repressor to the operator and readout of structural genes occurs whether lactose is present of not.
What is a non-inducible mutant?
This is a lacI mutation such that the repressor can no longer bond to the inducer lactose. The repressor therefore bind to the operator site preventing readout even when lactose is present.
Read through lac operon report before exam
Doooo it
Protein expression is being controlled.
What are the names of the proteins that are expressed all the time and what about the ones that are only produced in certain conditions.
Constitutive proteins: expressed all the time ie essential proteins
Regulated proteins-only produced in a certain condition ie required for an adaptation to a new environment. Or also called inducible proteins- because they are formed only when the inducer is present.
Regulation of protein expression
Regulated through metabolic pathways?
Here we shall focus on lactose metabolism
Lactose is imported into the cell and catabolised into the two monomeric units glucose and galactose
