2.4 Gene Regulation Flashcards
What are the three levels of gene regulation?
What is constitutive gene expression?
When a gene is transcribed at a relatively constant level under all conditions and cell types
What is regulated gene expression?
When a gene is transcribed only under certain cellular or environmental conditions, or in certain cell types
Why is gene expression even regulated?
A single celled organism such as bacteria must respond to changes in its environment. For example if a particualr carbon source becomes available then it may need particular enzymes to deal with that so those genes be turned on
Why is gene expression even needed in a multicellular organism?
- In a multicellular organism cells in a tissue like the gut can have different requirements.
- Absorptive cells will need to make lots of memebrane for microvilli and enzymes for digestion
- Goblet cells will need to make lots of mucus
- Stem cells will need proteins involved in cell division
- All of these things will require changes in gene expression
How do activators differ in prokaryotes vs eukaryotes for gene expression?
- In prokaryotes, activators bind near the promoter and help recruit RNA pol
- In eukaryotes activators can bind to enhancer sites a long way away from the promoter. DNA can loop over and bring them into contact with the initiation complex
What are the three ways a repressor can inhibit transcription?
- Repressor prevents the activator from binding (repressor usually dominant to activator if competitive for the same site)
- Repressor binds to the activator and inhibits its function
- Repressor directly binds and interferes with transcriptional machinery
How can transcription factors affect the chromatin state?
- In eukaryotes activators and repressors can recruit proteins/complexes that modify histones and remodel the chromatin structure
What is an example of an activator affecting chromatin state?
This activator has recruited a Histone Acetylase (HAT) which loosens the bond between histones and DNA making the TATA box more accessible.
What is an example of a repressor affecting chromatin state?
This repressor has recruited a chromatin remodelling complex which creates a condensed chromatin state, making the TATA box inaccessible
How do ligans regulate DNA binding?
Binding of a ligand to a transcription factor can change its conformation which can affect its binding to DNA
What are the different ways ligand affects the regulators?
- Ligand needed for activator to bind
- Ligand stops the activator binding
- Ligand needed for repressor to bind
- Ligand stops the repressor binding
What happens in the case where ligand is needed for ACTIVATOR to bind?
The removal of the ligand switches the gene off by removing the activator protein
What happens in the case where ligand stops activator binding?
Addition of ligand switches gene off by removing activator protein
What happens in the case where ligand is needed for the repressor to bind?
Removal of the ligand switches the gene on by removing respressor protein
How does ligand stop the repressor binding?
Which ligand regulation is positive and which is negative?
What is the consensus sequence in the promoter of a prokaryote?
- Analysis of many prokaryotic promoters reveals a consensus (i.e. common) sequence in -35 region and -10 region
- Conserved because they are the binding site of the sigma factor
What is the operator?
In prokaryotes it is the DNA region that controls the transcription of an adjacent gene. It is the binding site for a REPRESSOR, preventing RNA pol from binding
What are cis acting elements?
DNA sequences in the vicinity of the structural protein of a gene that are required for gene expression
What are trans acting factors?
Usually proteins that bind to the cis acting sequences to control gene expression
What are the carbon sources for E coli?
Glucose preferred carbon source as it can be easily utilised for energy through glycolysis and TCA cycle, or assimiliated into various compounds required for growth
What is the lactose metabolism?
Lactose disaccharide sugar that can be used as a carbon source but it must be converted to glucose and galactose by beta galactosidase
What is needed to use lactose as a carbon source?
- Need a lactose transporter protein (permease) to get it into the cell.
- Need the beta galactosidase enzyme to split it into glucose and galactose
What are the components of the lac operon?
How are the genes of the lac operon transcribed?
- The lac operon is transcribed as a single polycistronic RNA molecule.
- It encodes three genes/cistrons, which are translated into 3 proteins.
- Need multiple ribosomes translating it/
What is the LacI repressor?
- lacI encodes the lac repressor protein (LacI) which binds to the operator and inhibits transcription
- While lacI is adjacent to the lac operon it is not part of it
What happens in the absence of lactose (LacI)?
LacI repressor binds to the operator and blocks RNA polymerase, therefore repressing transcription
What happens in the presence of lactose? (LacI)
In the presence of lactose, the LacI repressor dissocaites from the operator and RNA polymerase can transcribe the operon
What is the CAP (catabolite activator protein) in the lac operon?
- The crp gene encodes CAP, a transcriptional activator.
- It helps RNA pol to bind to the promoter and is required for expression of the lac operon
What is the role of cAMP for CAP in the lac operon?
cAMP is a ligand for CAP. CAP can only bind DNA when cAMP is present
How does glucose regulate cAMP levels?
- We only want lac operator to be active when glucose is absent
- Glucose levels and cAMP levels are inversely related
- High levels of glucose REDUCE the levels of cAMP
- Low levels of glucose INCREASE levels of cAMP
- Glucose does this by repressing the activity of the enzyme adenylate cyclase which converts ATP to cAMP
