Lecture 26 Flashcards
Prokaryotes 2
Where is the operator located compared to the promoter
it is downstream to the promoter
why are operons useful/economical?
it puts all the enzymes and proteins involved in the same process under her control of a single promoter
polycistronic mRNA
multiple genes on a single mRNA stand, but they are translated independently
housekeeping genes
genes that are constantly expressed throughout the lifetime of the organism. eg ones in metabolic process
What does expressing the gene mean?
for the gene to show a phenotype. (DNA-mRNA-protien). Some do not to the protein stage.
Activators
they are proteins that increase the expression level of genes by binding to DNA, increasing activity of the RNA polymerase
Repressors
they are proteins that reduce the expression levels of genes by binding to DNA, decreasing the activity of the RNA polymerase
Effectors
they bind to an activator or repressor to alter the affinity the activator or repressor has for the gene. It can induce or repress gene expression
positive regulation
when an activator binds to DNA enhancing the activity of the RNA polymerase
negative regulation
when a repressor bind to DNA blocking the activity of the RNA polymerase
what is an Inducer
When an effector decreases the affinity of a repressor to DNA so gene expression occurs (negative regulation)
When an effector decreases the affinity of an activator a DNA…
it stops gene expression
What is a co-repressor?
When an effector increases the affinity of a repressor to DNA helping it stop gene expression (negative regulation)
When an effector increases the affinity of an activator to DNA…
it helps transcription occur. It cannot activate gene expression without the effector.
What genes encode repressors?
regulator genes
What is the lac operon?
it is 3 genes that make the enzymes to catalyze lactose into glucose and galactose when glucose is not present.
What is lacZ?
structural gene that encodes B-galactosidease
What is lacY?
structural gene that encodes galactoside permease and its used for transporting lactose across the bacterial membrane
What is lacA?
structural gene that encodes thiogalactoside transacetylase to export unwanted toxins that come into the cell from the permease activity
what is the lacI?
its a regulatory gene adjacent to the lac operon that makes the lac repressor protein LacI (lac inhibitor). it binds to the lac operator when lactose is absent
Negative regulation of the lac operon
When lactose is present, the B-gal turns it into glucose and galactose. It also changes some of it into its isomer allolactose which then acts like an inducer on the lac repressor. That inhibits the lac repressor from blocking the expression of the lac operon and gene expression occurs.
how does the repressor bind to the operon
it binds at 35 base pairs in a near perfect symmetry called the helix turn helix motif. It binds as dimers
Positive regulation of the lac operon
an effector called cAMP binds to the activator protein CRP (cAMP receptor protein) to increase affinity for the lac promoter. it increases expression because it distorts the DNA enough for RNA pol to bind more effectively
where does cAMP come from?
it is produce by adenylate cyclase
Why is cAMP produced when glucose is absent?
glucose inhibits adenylate cyclase
What are the 3 scenarios for lac operons
1) no lactose
2) lactose present, glucose absent
3) both lactose and glucose present
what happens when no lactose is present
the lac operon is off, no expression
what happens when lactose is present but glucose is not
high levels of expression because
- there’s no inhibition of adenlayte cyclase to make cAMP which will bind to CRP being a positive regulator and increase RNA pol activity
- the inducer allolactose will be produced and bind to the lac repressor so now expression occurs
what happens when both glucose and lactose are present
low levels of expression because
- glucose inhibits production of cAMP
- the allolactose will still be produced so the lac repressor will be removed
what happens when both glucose and lactose are gone
no expression
- the lac repressor is on the operon cause there no allolactose
- cAMP is high but that doesn’t matter
how is the trp operon an example of a negative regulation by a repressor in need of a co repressor?
Tryophan is the co repressor needed for its operon. When it is present, it binds to the repressor so it doesn’t make more. When it is absent, the repressor won’t be binding so transcription occurs
