Operon Flashcards
What does ChIP-seq measure?
Protein-protein interaction specifically
Define regulatory gene
Genes coding for product that control expression of other genes
Define operon
Unit of bacterial gene expression and regulation = includes structural genes and control elements in DNA recognized by regulatory gene products
Define cis-acting elements
Exerts their regulatory influence only on adjacent genes on the same chromosome
Examples = promoters, enhancers, silencers
Define trans-acting proteins
Regulatory proteins/molecules that can diffuse through the cell and exert their regulatory influence on genes located at distant chromosomal locations
Examples = transcription factors and RNA-binding proteins
Define negative transcriptional control
Repressor protein bind operator = preventing gene from being expressed
Define positive transcriptional control
Activator (TF) = required to bind at promoter To enable RNAp to initiate transcription
What are genes located adjacent rto each other transcribed into?
Polycistronic mRNA
What does the lac operator overlap?
Promoter at the start of the cluster (PO)
What doe lac I code for and have?
Lac repressor = has its own promoter and terminator
What genes does the lac repressor control
lacZYA
What controls the lac repressor?
Inducer
What is the natural inducer?
1,6-allolactose
What is the role of the inducer?
Converts repressor to form with lower operator affinity
This allows RNAp to initiate transcription
What is the lac operon gratuitous inducer?
IPTG = not metabolized
What is a gratuitous inducer?
Can induce the expression of a gene or set of genes in an organism, typically in a manner similar to a natural inducer
But without providing any benefit or nutritional value to the organism.
It is an inducer molecule that is not normally encountered or utilized by the organism in its natural environment.
What are the lac repressors binding sites?
Binds to operator DNA and inducer
Repressor dimers form a tetramer by interaction between what?
Interactions between oligomerization helices
What does the N-terminal DNA-binding domain consist of?
2 alpha-helical regions = helix-turn-helix
This binds to MAJOR groove of DNA
What is located at the core of the lac repressor?
Inducer-binding site
And reigons responsible for multimerization
Describe the operator the lac repressor binds to
Palindromic sequence of 26 bp
What does inducer binding to repressor cause?
Change in repressor conformation
Reducing affinity for DNA and releases it from the operator
What happens in the case of mutations in the operator (Oc)?
Constitutive expression of all 3 genes
Because operator is unable to be bound with repressor protein
Allowing RNAP to have unrestrained access to promoter
Oc mutation = cis-dominant, what does this mean?
Operator can control only the gene adjacent to it
These mutations are cis-acting and effect only those genes on the contiguous stretch of DNA
What happens in lacI- mutation?
Mutation that inactivates the lacI genes
Repressor cannot bind to the operator
Induction of a normal lacI+ gene can restore control even in the presence of a defective lacI- gene
Recessive
What is lacIs?
Super suppressor
Mutation in inducer-binding site of the repressor
Binds the operator and prevents transcription because cannot be removed
Uninducibility
What is lac-d?
Dominant
Mutations in DNA-binding site of the repressor
Repressor cannot bind the operator = constitutive
Why is lac-d dominant?
Mutant subunits can bind with normal subunits
Only requires one of the subunits to be mutant to block repressor function = cannot bind DNA
Explains dominant negative behaviour
Which operator does the lac repressor have the highest affinity for?
O1
What is the second layer of control for the lac operon?
Catabolite repression
What is catabolite repression?
Ability of glucose to prevent the expression of a number of genes
What is the other name for CAP?
cAMP receptor protein = CRP
Catabolite activator protein = CAP
What is the role of cAMP in regards to CRP?
cAMP converts inactive CRP to a form that binds the promoter
Assists RNAP in initiating transcription
How does glucose cause catabolite repression?
When glucose is high = low cAMP
Preventing:
cAMP activates a DIMER of CRP
CRP interacts with the CTD of the alpha subunit of RNAP to activate it
So RNAP not activated in the end
What are the lacC, P, O sites?
CAP binding site
Promoter = CAP-dependent
Operator = repressor binding site
What are the genes of the lac operon?
lacZ = beta-galacosidase, stains colonies blue
lacY = permease, transports lactose
lacA = transacetylase
How is the lac+ phenotype examined?
Nutrient agar containing IPTG and X-gal
X-gal cleaved by beta-galactosidase = blue
So if the lac+ is working = there should be blue because the enzyme is produced
