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Genetics CH 16: Control of Gene Expression in Bacteria
Terms in this set (46)
No: depends on simultaneous transcription and
Would you expect to see attenuation in
No: product of operon must affect translation
Would you expect to see attenuation in lac
repression is never complete; some transcription
is initiated even when repressor is active
Why do bacteria have attenuation? Shouldn't
repression at the trp operon be enough?
encode proteins that are used in metabolism, biosynthesis, or play a role in the cell
products, either RNA or proteins, interact with other DNA sequences and affect their transcription or translation
genes that are not regulated, continually expressed
DNA sequences that aren't transcribed but still play a role in regulation, affect the expression of sequences to which they're physically linked
regulates expression of structural genes by controlling transcription
binds to operator site to regulate the transcription of mRNA
regulator protein is a repressor - binds to DNA, turns off transcription
regulator protein is an activator - stimulates transcription
transcription normally off, must be turned on (induced)
transcription normally on, must be turned off (repressed)
change shape on binding to another molecule (ie. inducer)
binds to the repressor and makes it capable of binding to the operator
1. Z gene - beta galactosidase
2. Y gene - permease
3. A gene - galactoside transacetylase
three structural genes of the lac operon
regulator (repressor) binds to the operator and inhibits transcription by blocking the RNA pol from binding to the promotor
what happens in the lac operon when lactose is absent
some lactose is converted to allolactose which binds to the regulator protein, making it inactive and unable to bind to the operator allowing the structural genes to be transcribed and translated
what happens in the lac operon what lactose is present
simultaneous synthesis of several proteins stimulated by the inducer (allolactose)
bacteria possess two lac operons
results from positive control, bacteria prefer to use glucose, when glucose is present, genes metabolize other sugars are repressed
catabolite activator protein (CAP)
binds to a site ~22 nucleotides long and located within or slightly upstream of the lac genes promoer, binds to cAMP and then to DNA allowing RNA pol to more effectively bind to the promoter
1. Lactose Present
2. Glucose Low
two requirements for maximal transcription
transcription begins at the start site, but termination takes place prematurely, before RNA pol reaches the structural genes
secondary structure in the 5'-UTR of the trp operon (1+2 and 3+4 structure)
2+3 secondary structure
control gene expression by binding to sequences of mRNA and inhibiting translation
regulatory sequences where molecules can bind and affect gene expression by influencing the formation of compact secondary structures (consisting of a base stem and several branching hairpins) in the mRNA
Gene regulation allows for biochemical and internal flexibility while maintaining energy efficiency by the bacterial cells.
Why is gene regulation important for bacterial cells?
(1) Alteration of the gene structure at the DNA level
(2) Transcriptional regulation
(3) Regulation at the level of mRNA processing
(4) Regulation of mRNA stability
(5) Regulation of translation
(6) Regulation by posttranslational modification of the synthesized protein
Name six different levels at which gene expression might be controlled.
In catabolite repression, the presence of glucose inhibits or represses the transcription of genes involved in the metabolism of other sugars, only enyzmes involved in metabolism of glucose are synthesized. Operons that exhibit catabolite repression are under the positive control of catabolic activator protein (CAP). For CAP to be active, it must form a complex with cAMP. Glucose affects the level of cAMP. The levels of glucose and cAMP are inversely proportional—as glucose levels increase, the level of cAMP decreases. Thus, CAP is not activated.
What is catabolite repression? How does it allow a bacterial cell to use glucose in preference to other sugars?
Antisense RNA molecules are small RNA molecules that are complementary to other DNA or RNA sequences and that form RNA-protein complexes. In bacterial cells, antisense RNA molecules can bind to a complementary region in the 5' UTR of a mRNA molecule, blocking the attachment of the ribosome to the mRNA and stopping translation or they pair with specific regions of the mRNA and cleave the mRNA stopping translation.
What is antisense RNA? How does it control gene expression?
At riboswitches, regulatory molecules bind and influence gene expression by affecting the formation of secondary structures within the mRNA molecule. The binding of the regulatory molecule to a riboswitch sequence may result in repression or induction. Some regulatory molecules bind the riboswitch sequence and stabilize a terminator structure in the mRNA, which results in premature termination of the mRNA molecule. Other regulatory molecules bind riboswitch sequences resulting in the formation of secondary structures that block the ribosome binding sites of the mRNA molecules, thus preventing translation initiation. In induction, the regulatory molecule acts as an inducer, stimulating the formation of a secondary structure in the mRNA that allows for transciption or translation to occur.
How do riboswitches control gene expression?
Because the blob operon is transcriptionally inactive in the presence of B, gene Smost likely codes for a repressor protein that requires compound B as a corepressor. The data suggest that the blob operon is repressible because it is inactive in the presence of compound B, but active when compound B is absent.
The operon is controlled by a regulatory gene S. Normally, the enzymes are synthesized only in the absence of compound B. If gene S is mutated, the enzymes are synthesized in the presence and in the absence of compound B. Does gene S produce a repressor or an activator? Is this operon inducible or repressible?
Condition 1 will result in the production of the maximum amount of β-galactosidase. For maximum transcription, the presence of lactose and the absence of glucose are required. Lactose (or allolactose) binds to the lac repressor reducing the affinity of the lac repressor to the operator. This decreased affinity results in the promoter being accessible to RNA polymerase. The lack of glucose allows for increased synthesis of cAMP, which can complex with CAP. The formation of CAP-cAMP complexes improves the efficiency of RNA polymerase binding to the promoter, which results in higher levels of transcription from the lac operon.
Under which of the following conditions would a lac operon produce the greatest amount of β-galactosidase? The least? Explain your reasoning. Condition 1: lactose present, no glucose
Condition 2: no lactose, glucose present
Condition 3: lactose and glucose present
Condition 4: no lactose or glucose
The operator region is most likely, if the mutation prevents the lac repressor from binding to the operator, transcription will never be inhibited and expression will be continuous.
A mutant strain of E. coli produces β-galactosidase in the presence and the absence of lactose. Where in the operon might the mutation in this strain occur?
The lacI gene encodes the lac repressor protein, which can diffuse within the cell and attach to any operator. It can therefore affect the expression of genes on the same or different molecules of DNA. The lacO gene encodes the operator. The binding of the lacrepressor to the operator affects the binding of RNA polymerase to the DNA, and therefore affects only the expression of genes on the same molecule of DNA
Explain why mutations in the lacI gene are trans in their effects, but mutations in the lacOgene are cis in their effects.
At which level of gene regulation does attenuation occur?
If the ribosome does not bind to the 5' end of the mRNA, then region 1 of the mRNA 5' UTR will be free to pair with region 2, thus preventing region 2 from pairing with region 3 of mRNA 5' UTR. Region 3 will be free to pair with region 4, forming the attenuator or termination hairpin. Transcription of the trpstructural genes will be terminated. Essentially, no gene expression will occur.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes?: A mutation that prevents the binding of the ribosome to the 5′ end of the mRNA 5′ UTR
If alanine codons have replaced tryptophan codons, then under conditions of high alanine, the stalling of the ribosome will not occur. The attenuator will form, stopping transcription. The ribosome will stall when alanine is low, so transcription of the structural genes will occur only when alanine is low.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? A mutation that changes the tryptophan codons in region 1 of the mRNA 5'UTR into codons for alanine
An early stop codon will result in the ribosome "falling off" region 1, allowing it to form a hairpin structure with region 2. Transcription will not occur because regions 3 and 4 are now free to form the attenuator.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? A mutation that creates a stop codon early in region 1 of the mRNA 5' UTR
If region 2 of the mRNA 5' UTR is deleted, then the antiterminator cannot be formed. The attenuator will form and transcription will not occur.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? Deletions in region 2 of the mRNA 5' UTR
The trp operon mRNA 5' UTR will be unable to form the attenuator if region 3 contains a deletion. Attentuation or termination of transcription will not occur, resulting in continued transcription of the trp structural genes.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? Deletions in region 3 of the mRNA 5' UTR
Deletions in region 4 will prevent formation of the attenuator by the 5' UTR mRNA. Transcription will proceed.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? Deletions in region 4 of the mRNA 5' UTR
For the attenuator hairpin to function as a terminator, the presence of a string of uracil nucleotides following region 4 in the mRNA 5' UTR is required. The deletion of the string of adenine nucleotides in the DNA will result in no string of uracil nucleotides following region 4 of the mRNA 5' UTR. No termination will occur, and transcription will proceed.
What will the most likely effect of each of these mutations be on transcription of the trp structural genes? Deletion of the string of adenine nucleotides that follows region 4 in the 5' UTR
Mutations that disrupt the formation of the antiterminator will increase termination by the attenuator. Such disruptions could be caused by a deletion in region 2 that prevents region 2 from pairing with region 3. Mutations in region 1 could also affect the antiterminator if the mutations prevented the ribosome from stalling at the adjacent trpytophan codons within region 1
Some mutations in the trp 5' UTR region increase termination by the attenuator. Where might these mutations occur and how might they affect the attenuator?
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