L6-Regulation of Gene Expression Flashcards
Which is more rapid, regulation of activity of an enzyme or regulation of synthesis? Why?
Regulating the activity of an enzyme is more rapid than regulation of synthesis because it takes some time before A) enough proteins are present to affect metabolism (after synthesis) or B) the proteins are sufficiently diluted so they do not affect metabolism.
Why are prokaryotes able to respond so quickly to environmental changes?
The half-life of a typical mRNA in prokaryotes is short (few minutes at best) and mRNAs can be removed rapidly when they are no longer needed/prevents production of unneeded proteins.
What is protein domain?
Regions of protein with a specific structure and function.
Why are some interactions of proteins with DNA specific to certain DNA sequences?
Binding proteins must be able to bind in the major groove of DNA (because it is large enough) and simultaneously with several nucleotides. This entails high specificity of proteins binding to base sequences.
How does a repressor work to inhibit the synthesis of a specific mRNA?
A corepressor (ex. Arginine) binds to a specific allosteric repressor protein. Conformation is altered and the protein becomes active/binds to a specific region of DNA near the promoter of the gene near the operator. Since the operator is downstream of the promoter, the repressor physically blocks transcription because RNA polymerase cannot bind nor proceed and the genes in the operon/mRNA are not transcribed.
Version 2: Repressor is active in absence of inducer/blocks transcription. When an inducer is present, it combines with the protein and inactivates it so that transcription can proceed.
Give an example and explain positive control of enzyme induction?
Positive control-the regulator protein is an activator that activates the binding of RNA polymerase to DNA.
Ex. Catabolism of the sugar maltose in E. coli. Maltose activator protein binds to DNA (at activator binding site), and allows RNA polymerase to begin transcription by helping it frecognize the promoter. The activator protein cannot bind to DNA unless it first binds to maltose, the inducer.
Explain how the lac operon is both positively and negatively controlled.
The lac operon is negatively controlled when there is a better carbon source available (mainly glucose). Expression of lac operon is prevented by the lactose repressor. When there is lactose available and it is the best carbon source, the lac operon is positively controlled (catabolite repression is abolished) and the lac operon is expressed/cells grow on lactose. Lactose/another molecule must be capable of acting as an inducer. Also, cyclic AMP must be high enough forCRP protein to bind to the CRP binding site.
Explain the interaction of MCPs, Che proteins and the flagellar motor and how that leads to chemotaxis.
- • methyl-accepting chemotaxis proteins-sensory transmembrane proteins that can sense certain compounds. Binds directly or indirectly (through interactions with periplasmic binding proteins) to trigger interaction with CheA and CheW
- • Che proteins-autophosphorylate then passes phosphorylation to CheY which is a response regulator that controls flagellar rotation. CheY governs the direction of rotation of the flagellum. When phosphorylated, CheY binds to motor to induce clockwise rotation and tumbling. When unphosphorylated, CheY does not bind and the flagellum rotates counterclockwise/run.
- • Methylation-Methylated MCPs are more responsive to repellents and unmethylated MCPs respond to attractants. During a run, MCPs become increasingly methylated so if there is a high but constant level of attractant, the cell begins to tumble. CheB becomes phosphorylated and demethylates MCPs to reset the receptors. (Repellent system is opposite).
Define an autoinducer and explain how they are used in quorum sensing.
An autoinducer is a signal molecule that diffuses freely across the cell envelope . It only reaches high concentration in cells of there are many cells nearby making the same autoinducer. Inside the cell, the autoinducer binds to a specific activator protein and triggers transcription of specific genes.
Describe RNA regulation by antisense RNA.
Antisense mRNAs (~100 nucleotides) have regions of complementary sequence (~30 nucleotides) and base pair with other mRNA to prevent its translation. These are degraded by ribonucleases to remove the mRNA and prevent synthesis of new protein molecules. It is also believed that mRNA can enhance translation of target mRNA by preventing the native mRNA from forming a secondary structure that prevents translation.
How do riboswitches control gene expression?
Riboswitches are RNA molecules that resemble repressors and activators in binding metabolites and regulating gene expression. Many regulate translation of RNA. A metabolite binds to the riboswitch which activates it to block mRNA translation (ex. Binding to A site of ribosome).
Describe the mechanisms of bacterial attenuation.
Attenuation is a form of transcriptional control that functions by premature termination of mRNA synthesis (after initation of transcription).
