Bacteria
- Coordinately controlled genes clustered in operon regulated by one promoter
- Prokaryotic regulation mostly at transcriptional level
- Prokaryotes don’t have RNA splicing or chromatin modification
Operon
- “Unit of DNA that contains functionally related genes that can be coordinately controlled by “on/off” switch”
- Benefit of Operons: better coordination and control → can regulate cluster of functionally related genes with single on/off switch
- Promoter: sequence of DNA to which RNA polymerase attaches to begin transcription
- Operator: DNA site which binds to a regulatory protein that switches operon on/off to either block or promote RNA polymerase and regulate gene expression
- Structural genes: contains coding DNA→ sequences that code for various related protein subunits that direct production of specific end product
- Enzyme made like Tryptophan can accumulate and inhibit own production by acting as repressor protein and blocking operator
- Regulatory gene: lies outside operon region → produces a regulatory protein that binds to operator region and controls whether RNA polymerase can begin transcription
- Regulatory proteins are allosteric, can be one of two kinds
- Repressor protein: blocks attachment of RNA polymerase to promoter region
- Stops transcription & translation
- Activator protein: promotes attachment of RNA polymerase to promoter region
- Positive regulation because they must be active in order for transcription to occur
6. Corepressor: small molecule that binds to and activates a repressor protein to switch an operon off
Types of Regulation
Negative Regulation: Repressible and Inducible Operons
- Repressible operon: transcription usually on but can be inhibited when repressor binds to it
- Inducible Operon: transcription usually off but can be turned on when inducer binds to and inactivates repressor protein
- Both involve negative gene regulation bcuz operons are switched off by active form of repressor protein
- In general repressible operons are associated with genes that regulate anabolic pathways while inducible operons are associated with catabolic pathways.
Positive Gene Regulation
- Gene regulation positive bcuz activator regulatory protein directly interacts with operon to increase transcription
- Example: CAP
- CAP is activator protein which becomes active when cAMP binds to it → CAP attaches to promoter → increases RNA pol affinity for lac promoter → increases transcription and directly stimulates gene expression
- Glucose lvls high → cAMP lvls are down → CAP is inactive
- Glucose lvls low → cAMp lvls are high → CAP is activated
3 Examples of Gene Regulation
- Lac operon: controls breakdown of lactose
- No lactose present: In the absence of lactose, the repressor switches off the operon by binding to the operator.
Lactose Present: When lactose is present, and the bacteria needs to break down to digest it, allolactose (an isomer of lactose) acts as an inducer by binding to the repressor and inactivating it → repressor cannot block the operator, and RNA polymerase can bind to it and begin to transcribe the proteins needed to digest lactose.
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- Enzymes operon makes are inducible enzymes and operon is inducible operon
- Bcuz repressor protein is involved → negative regulation
- Trp Operon: regulatory gene produces inactive repressor that does not bind to operator → RNA pol can transcribe genes to make amino acids for enzyme
- When amino acid in environment → cell doesn’t need to make it → amino acid acts as a corepressor by binding to and activating the repressor → repressor binds to operator → prevents transcription
- Produced enzymes are repressible enzymes and operon is repressible operon
- Bcuz there is repressor protein → negative regulation.
- Glucose repression: 2nd regulatory process that influences the lac operon. Glucose is preferred over lactose → lactose only present → process enhances break down of lactose
- Uses activator regulatory protein, CAP, that is activated by cAMP → positive regulation
Eukaryotic Gene Expression Regulation
- Eukaryotic gene expression regulation is more complicated because…
- Multicellularity: requires different gene regulation for diff cell types
- Chromosome complexity: chromosomes are more complex bcuz of their larger size and organization with histone proteins
- Some metabolic processes require activation of multiple genes, each located on different chromosomes → requires a more sophisticated system of coordination
- Uncoupling of transcription and translation: Allows for a greater range to control gene expression.
- Genes are expressed when their nucleotide sequence are transcribed to produce RNAs
Eukaryotes: Coordinately Controlled Genes
- Operons not used in eukaryotes
- Genes co-expressed are scattered over different chromosomes and coordinate gene expression & metabolic activity depends on every gene having same transcription factors and combination of control elements
- Transcription factors in nucleus bind to control elements → promote simultaneous transcription of genes
- Coordinate gene regulation often occurs in response to chemical signals from outside the cell, either steroid or protein hormones that activate transcription factors
- Steroid hormones act directly
- Protein hormones act indirectly via a signal transduction pathway
- Genes with same set of control elements are activated by same chemical signals