Quiz 5 Flashcards
Regulation of gene expression
Controlling any step (transcription and translation)
How is regulation carried out in bacteria vs eukaryotes
Bacteria: primarily during transcription
Eukaryotes: more complex; transcription, translation, and post-translation
Dynamic
Some genes switched off and some on; different genes can be expressed at different levels depending on need; the same gene can be expressed differently in different cells
Constitutive genes
Always switched on
Inducible genes
Normally switched off but can be expressed if stimulated by an inducer/positive regulator
Repressible genes
Normally switched on; expression occurs unless a repressor/negative regulator switches off expression
Positive regulation
Stimulates transcription; carried out by trans-acting factors called activators and occurs in both repressible and inducible genes
Negative regulation
Inhibits or downregulates transcription; carried out by trans-acting factors called repressors and occurs in both repressible and inducible genes
Operon
A group of adjacent genes that are transcribed from a shared promoter (mRNA for these genes is thus polycistronic)
Proteins encoded by operon
Have related biological functions and the proteins work together; as such, the cell controls a specific biological function by regulating the shared promoter of the genes in said operon
Components of an operon
A group of structural genes (encode proteins), regulatory region (cis-acting site) a.k.a. regulatory site that consists of a promoter and an operator, and regulatory genes (repressor gene that encodes for a repressor protein that controls expression of operon by binding to operator)
Controlling the ability of RNA polymerase to initiate transcription from the shared promoter:
Controls expression of all the structural genes of the operon
E. Coli’s preferred food and second best option
Glucose, then lactose and other sugars
Function of lac operon
An inducible operon that can encode necessary proteins for using lactose as food
Structure of lac operon
Three structural genes (Z for B-galactosidase, Y for permease, and A for transacetylase), regulatory site, and the regulatory gene (lacl, the repressor)
Beta-galactosidase
Concerts lactose into glucose and galactose
Permease
Involved in lactose impoet into the cell
Transacetylase
Removes toxic byproducts
Regulatory site of lac operon
RNA polymerase (trans-acting factor) binds to promoter (cis-acting site) and repressor protein (trans-acting factor) binds to operator (cis-acting site)
Regulatory gene of lac operon
Called I, encodes the lac repressor protein
Regulation of the lac operon involves:
Negative regulation by the repressor protein, induction by lactose, and positive regulation by cAMP-CAP
Repression of lac operon
Repressor protein (I) bonds to operator in absence of lactose and blocks RNA polymerase from transcribing the structural genes
Constitutive mutations
Cause the repressor protein to not bind to the operator for whatever reason so there is no negative regulator present and the lac operon is always expressed
Induction of lac operon
Lactose binds to repressor protein and changes its 3D shape (an allosteric change), making it unable to bind to the operator; since RNA polymerase is no longer blocked it can transcribe the structural genes
Note: lactose inactivates all repressors within the fell; furthermore, if repressor protein is mutated so lactose can’t remove it or change it, it is permanently bound to the operator and the gene is permanently repressed
Super repressed
Cannot be expressed at all
If glucose and lactose are present at the same time
The lac operon is practically shit off with catabolite repression, which involves catabolite-activiating protein (CAP); the enzyme adenyl cyclase makes cAMP, and is shut off by glucose–if glucose levels are low, lots of cAMP is made to bind to CAP, a compound that can bind to the lac promoter at the CAP-binding site next to RNA polymerase
Cooperative binding
cAMP-CAP bound to the promoter enhances RNA polymerase binding to the promoter, exerting positive control in expression
Catabolite repression in the presence of glucose
Glucose shuts adenyl cyclase off so no cAMP is made and there no cAMP-CAP is formed to bind to the promoter and push RNA polymerase
Tryptophan operon
Encodes enzymes to make tryptophan (amino acid); repressible operon
Structure of the trp operon
Five structural genes that encode the enzymes that make tryptophan (E, D, C, B, and A), the regulatory region with the leader sequence and the attenuator, the promoter, and the operator, as well as the regulatory gene that encodes the repressor protein
When trp operon is expressed:
The repressor protein is inactive
Tryptophan regulation
IF levels become high, tryptophan binds to the inactive repressor protein causing an allosteric change that activates the repressor protein, switching off the trp operon
Corepressor
Required for repressor protein to bind to operator; ex: tryptophan
Trp operon expression
Low levels of tryptophan in a cell cause the repressor protein to be made; it cannot bind to the operator in its 3D shape, so the gene is expressed
Trp operon repression
High levels of tryptophan in the cell means tryptophan binds to the repressor protein, changing its 3D shape so it can bind to the operator and thus repress the gene
Trp operon attentuation
AN additional mechanism of repression of the trp operon in response to high levels of tryptophan in the cell by which the leader sequence is transcribed and THEN the trp operon is repressed; the trp opwron leader sequence contains a short coding sequence of 14 amino acids and two adjacent tryptophan codons in the first leader sequence
Regions 2, 3, and 4 can participate in an RNA hairpin formation, as it contains a stretch of U nucleotides called the attenuator
If tryptophan levels are low, regions 2 and 3 form antiterminator hairpin when the ribosome stalls at the tryptophan codons and transcription of the operon can take place; however, if levels are high then the ribosome doesn’t stall, so regions 2 and 3 don’t form a hairpin and regions 3 and 4 instead form a hairpin called a terminator hairpin that stops transcription
