chapter 7 review Flashcards
At how many levels can microbes control gene expression?
Several levels, including DNA, RNA, and protein stages.
What is the first level of gene expression control in microbes?
Alteration of the DNA sequence (e.g., mutations, rearrangements).
What is the second level of control in microbial gene expression?
Transcriptional control – regulating when and how genes are transcribed into mRNA.
What is the third level of gene expression control?
Control of mRNA stability – determining how long mRNA transcripts remain intact.
What is the fourth level of gene regulation?
Translational control – regulating how efficiently mRNA is translated into proteins.
What is the fifth level of gene expression control?
Posttranslational control – modifying proteins after translation (e.g., folding, cleavage, phosphorylation).
Why do microbes use multiple levels of gene expression control?
Different levels provide flexibility and efficiency, allowing microbes to respond quickly to environmental changes.
What do repressors do?
They bind to regulatory DNA sequences and prevent transcription of target genes.
Do repressors always act alone?
No, some repressors must first bind a small ligand to function.
What do activators do?
They bind to regulatory DNA sequences and stimulate transcription of target genes.
Do activators require anything to function?
Most activators must bind a small ligand before they can activate transcription.
What’s the role of small ligands in transcriptional regulation?
They help regulatory proteins (repressors or activators) change shape or function to bind DNA.
Who proposed the idea that genes could be regulated?
Jacques Monod and François Jacob in 1961.
What observation led Monod and Jacob to propose gene regulation?
In E. coli, enzymes for lactose metabolism were inducible, but enzymes for glucose metabolism were constitutive.
What does inducible mean in the context of gene expression?
The gene is turned on only when needed, in response to a specific substrate (like lactose).
What does constitutive mean in gene expression
The gene is always expressed, regardless of environmental conditions (like those for glucose metabolism).
What major award did Monod, Jacob, and Lwoff win?
The 1965 Nobel Prize in Physiology or Medicine.
What other topic did André Lwoff study that contributed to the Nobel Prize?
Phage lysogeny (how viruses integrate into bacterial genomes).
What is the lac operon in Escherichia coli?
A set of genes (lacZYA) responsible for the transport and metabolism of lactose.
What is significant about the lac operon?
It was the first gene regulatory system ever described.
What does the lacZ gene encode?
β-galactosidase, an enzyme that breaks down lactose into glucose and galactose.
What does the lacY gene encode?
Lactose permease, a membrane protein that transports lactose into the cell.
What does the lacA gene encode?
Thiogalactoside transacetylase, whose exact role in lactose metabolism is less clear but may help detoxify byproducts.
When is the lac operon activated?
When lactose is present and glucose is absent (or low).
What happens to the lac operon in the absence of lactose?
It is repressed and not transcribed.
Which protein is responsible for repressing the lac operon when lactose is absent?
The LacI repressor.
How does the LacI repressor inhibit transcription?
It binds as a tetramer to the operator region, blocking RNA polymerase from forming the open complex.
What is the result of LacI binding to the operator?
Transcription is prevented, so the lacZYA genes are not expressed.
What happens to the lac operon in the presence of lactose?
It becomes induced (turned on), allowing transcription of lacZYA genes.
What enzyme is involved in the initial processing of lactose?
β-galactosidase (LacZ)
What does LacZ do at low levels when lactose is present?
It cleaves and rearranges lactose to produce allolactose, the inducer.
What is the role of allolactose in lac operon regulation?
It binds to the LacI repressor, causing a shape change that reduces its binding to the operator.
What are the structural genes in the lac operon?
lacZ, lacY, and lacA – involved in lactose metabolism.
What happens when LacI is no longer bound to the operator?
RNA polymerase can now bind and initiate transcription of the lacZYA operon.
What does the lacZ gene encode?
β-galactosidase, which splits lactose into glucose and galactose.
What is the promoter site in the lac operon?
The DNA sequence where RNA polymerase binds to begin transcription.
What is the operator site?
A cis-acting DNA region where the repressor binds to block transcription.
What is the lacI gene?
A gene located upstream that encodes the trans-acting repressor protein.
What is the function of the repressor in the lac operon?
It binds to the operator and prevents transcription when lactose is absent.
What is an inducer in the context of the lac operon?
A molecule (e.g., allolactose) that binds to the repressor, preventing it from binding to the operator and allowing transcription.
What is required for maximum expression of the lac operon?
The presence of cAMP and the cAMP receptor protein (CRP).
What does the cAMP-CRP complex do in the lac operon?
The cAMP-CRP complex binds to the promoter and interacts with RNA polymerase to increase transcription initiation.
How does the cAMP-CRP complex affect RNA polymerase?
It enhances the interaction between RNA polymerase and the promoter, leading to increased transcription of the lac operon.
What does cAMP bind to in the lac operon?
cAMP binds to the CRP (cAMP receptor protein), forming the active cAMP-CRP complex.
When is the cAMP-CRP complex most active?
When glucose levels are low, resulting in higher cAMP concentrations.
What is catabolite repression?
It is the repression of an operon enabling the catabolism of one nutrient when a more favorable nutrient (often glucose) is present.
How does glucose affect the lac operon?
Glucose represses the lac operon, preventing the expression of genes needed for lactose metabolism.
What is diauxic growth?
A biphasic growth pattern observed when a culture grows on two carbon sources, with glucose being used first, followed by lactose after glucose is exhausted.
Why does glucose repression occur in the lac operon?
In the presence of glucose, cAMP levels are low, so the cAMP-CRP complex cannot form, leading to decreased activation of the lac operon.
How does the presence of glucose lead to low cAMP?
Glucose inhibits the production of cAMP, preventing the cAMP-CRP complex from stimulating transcription of the lac operon.
What is the order of carbon source usage in diauxic growth?
The cell first uses glucose, then switches to lactose once glucose is depleted.
How does glucose transport affect the lac operon?
It causes catabolite repression by inhibiting LacY permease
What is inducer exclusion?
Glucose inhibits LacY permease, preventing lactose from entering the cell.
How does inducer exclusion affect the lac operon?
If lactose can’t enter, the lac operon is not induced.
What does LacY permease do?
It transports lactose into the cell.
How does the phosphotransferase system (PTS) cause catabolite repression?
It inhibits LacY permease, blocking lactose entry.
What happens if lactose can’t enter the cell?
: The lac operon remains repressed.
What does the trp operon encode?
Enzymes involved in tryptophan production.
How is the trp operon regulated?
By tryptophan levels: excess tryptophan acts as a corepressor.
What happens when tryptophan binds to TrpR?
It activates TrpR, forming the holorepressor, which binds to the operator.
What is the effect of the holorepressor binding to the operator
It blocks RNA polymerase, repressing transcription of the trp operon.
How much does repression lower the expression of the trp operon?
Repression reduces expression by about 100-fold.
How do bacteria coordinate the activation of genes with different functions?
By regulating the synthesis or activity of sigma factors.
What is a regulon?
A set of genes and operons with related functions, controlled by a single repressor or activator.
What is the role of a sigma factor?
It directs RNA polymerase to transcribe genes in a regulon.
Why do bacteria regulate sigma factors?
To coordinate the expression of multiple genes and operons needed for survival.
What is the role of the sensor kinase in a two-component regulatory system?
it binds to an environmental signal and phosphorylates itself.
What does the response regulator do in the system?
It takes phosphate from the sensor and binds the chromosome to alter transcription.
Where is the sensor kinase located?
In the cell membrane.
What happens when the response regulator is phosphorylated?
It modifies transcription of genes based on the environmental signal.
What does the two-component system help bacteria do?
Sense and respond to changes in the external environment
What is quorum sensing?
A process by which bacteria coordinate gene expression based on their population density.
What is AHL in the context of quorum sensing?
Acyl-Homoserine Lactone (AHL) is a type of autoinducer used by Gram-negative bacteria for quorum sensing.
How do AHL molecules function in quorum sensing?
AHL molecules are secreted by bacteria and, when their concentration reaches a certain threshold, they trigger gene expression for coordinated behaviors.
What are regulatory RNAs?
Regulatory RNAs are non-coding RNAs that control gene expression by interacting with mRNA, proteins, or other RNAs.
What is a riboswitch?
A riboswitch is a segment of mRNA that can bind small molecules, causing a structural change that affects gene expression.
How do riboswitches control gene expression?
They change mRNA structure, affecting transcription or translation.
What is attenuation?
Premature termination of transcription based on environmental conditions.
How does attenuation work in the trp operon?
It terminates transcription when tryptophan levels are high.
How do riboswitches affect translation?
They control translation by exposing or hiding the ribosome-binding site.
What are regulatory RNAs?
Non-coding RNAs that do not translate into proteins but regulate gene expression.
What types of non-coding RNAs are included as regulatory RNAs?
rRNAs, tRNAs, signal recognition particle RNA, and small RNAs.
What is the size of small RNAs?
They are 40–400 nucleotides long.
How do small RNAs regulate gene expression?
By base-pairing with mRNA, blocking translation or affecting mRNA stability.
Why can’t double-stranded RNA be translated?
Double-stranded RNA is recognized as foreign and cannot be used for translation.
What is the role of riboswitches in biosynthetic pathways?
They regulate gene expression based on the presence of specific metabolites.
Which vitamins are regulated by riboswitches in E. coli?
Cobalamin B12, tetrahydrofolate (folic acid), and thiamine.
Which amino acids are controlled by riboswitches in E. coli?
Glutamine, glycine, lysine, and methionine.
Which nitrogen bases are regulated by riboswitches in E. coli?
Adenine and guanine (purine bases).
What are some other metabolites regulated by riboswitches?
FMN, SAM, glucosamine 6-phosphate, cyclic di-GMP, and molybdenum cofactor.
What is attenuation?
A regulatory mechanism where translation of a leader peptide affects the transcription of downstream genes.
In the trp operon, what region is involved in attenuation?
The attenuator region of the trp operon.
What does the attenuator region contain in the trp operon?
It contains two trp codons and can form stem-loop structures.
How do stem-loop structures in the trp operon affect transcription?
They cause premature termination of transcription if the tryptophan levels are sufficient.
What is feedback inhibition?
When the end product of a pathway inhibits an enzyme early in the pathway.
How does feedback inhibition work?
The end product reduces enzyme activity, stopping overproduction.
Where does feedback inhibition occur?
In metabolic pathways.
What role does the end product play in feedback inhibition?
It signals to decrease enzyme activity when levels are high.
What is an operon?
A group of 2+ genes controlled by 1 promoter.
What is a regulon?
A set of multiple operons regulated by a single regulatory protein.
What is global control?
Regulation of many unrelated genes in response to environmental changes.
Catabolism
Production of an enzyme when
the substrate is present
Anabolism
Enzyme production is repressed
unless the product is absent
