Term
| What level does most of the prokaryotic gene regulation occur at? |
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Definition
| at the level of transcription |
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Term
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Definition
A segment of DNA containing a set of contiguous genes coding for enzymes and other proteins closely related to metabolic function.
An operon includes control elements which are promoters and the regulatory gene.
It also includes the structural genes which are transcribed to produce a polygenic mRNA |
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Term
| What are the main themes behind the operon? |
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Definition
All genes on the genome are not expressed simultaneously.
Primarily, environmental stimuli determine what promoters (and thus which operons) are available at any given time (e.g., nutrients or lack of, procreation, survival). |
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Term
| How does a catabolic operon function? |
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Definition
The promoter for an operon encoding enzymes to metabolize an environmental nutrient (catabolic operon) becomes available only when that nutrient is present.
This response is called enzyme induction.
Default state of catabolic operon is off.
Enzymes synthesized in response to their substrates are said to be induced.
A common example of this involves enzymes that convert complex sugars into glucose.
Lactose is an inducer of the lac operon. |
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Term
| How does an anabolic operon work? |
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Definition
The lack of a nutrient in the environment can determine what promoters (and operons) are available.
The promoter for an operon encoding enzymes to synthesize an essential nutrient (anabolic operon) becomes unavailable when that nutrient is present in the environment.
In this case, the operon is said to be repressed.
Default state of operon is on.
Essential elements like amino acids, nucleotides, and cofactors fall into this category, and are called corepressors.
The classic example is the tryptophan (trp) operon. |
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Term
| What are control elements? |
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Definition
Closely associated DNA sequences that control the transcription of the genes of an operon.
Examples are promoter, operator, regulatory gene, the CAP (CRP) binding site (CBS), the attentuator site |
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Term
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Definition
| Region to which the RNA polymerase binds to initiate transcription. It is present on both types of operons. It is upstream of the first transcribed base. |
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Term
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Definition
It is the binding site for the repressor protein. When this area is bound it blocks transcription.
Found on both types of operons.
Lies between the promoter and 1st transcribed base. Can overlap with either the promoter or the 1st base. |
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Term
| What is the regulatory gene? |
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Definition
| It codes for repressor protein mRNA. It can be separated from the operon but it must be close. It contains its own separate promoter |
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Term
| What is the CRP (CAP) binding site? |
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Definition
This is where the cAMP-bound CRP binds and stimulates initiation of transcription by RNA polymerase.
It is ONLY found on catabolic operons.
It exists adjacent to or overlaps the promoter. |
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Term
| What is the attentuator site? |
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Definition
A region found in some anabolic operons coding for enzymes to synthesize amino acids.
It lies within the transcriptional unit and it is normally transcribed and translated.
Ribosome binding at the newly transcribed attentuator mRNA leads to termination of further transcription before the polymerase gets to the first gene.
Simplified - attentuator site is a DNA region that terminates transcription after itself has been transcribed |
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Term
| What genes are adjacent are to the lac operon? |
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Definition
Lac A, Lac Y, and Lac Z.
Lac Y is important because it codes for a permease, which is a protein that occurs in the cell membrane and is important for lactose transport across the membrane. |
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Term
| How does induction of the lac operon work? |
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Definition
Induction of the three lactose genes occurs during transcription initiation.
Without lactose present, transcription of the gene cluster happens at a very low level.
A single mRNA containing the three genes is transcribed.
The transcript has a very short half-life (3 minutes) so that expression of the operon can be changed quickly. |
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Term
| How is the lac operon repressed? |
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Definition
LacI codes for the repressor protein.
LacI is nearby upstream of the gene cluster.
transcription is not regulated and it always occurs at a low rate independent of what is going on with the cell.
the repressor protein has a strong affinity for both the operator and lactose.
Lactose can bind the repressor protein when it is free or when it is bound to operator and prevents repression. |
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Term
| Which sites in the operon have palindromic sequences? |
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Definition
The operator and the CAP (CRP) binding site.
The promoter can't be palindromic because RNA polymerase binding needs to happen in only one direction. |
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Term
| How is glucose related to the lac operon? |
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Definition
Since E. coli prefers to use glucose as opposed to lactose, it will metabolize glucose 1st.
Glucose interferes with the induction of the lac operon and this is called catabolite repression. |
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Term
| What is the relation of glucose and cAMP to the lac operon? |
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Definition
cAMP binds to CAP (CRP) and allosterically modifies it.
the cAMP-CAP complex binds to the CAP binding site and facilitates the binding of RNA polymerase to the promoter and transcription is enhanced.
the presence of glucose causes a decrease in cAMP and thus a decrease in the cAMP-CAP complex
without glucose cAMP levels are high |
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Term
| What is the transcription rate of the lac operon when there is no glucose or lactose present. |
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Definition
| The transcription rate is slow because there is rate set even when there isn't any induction or repression. |
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Term
| What is the transcription rate of the lac operon when glucose is present but lactose isn't? |
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Definition
| The transcription rate is at the lowest level possible (not absolutely off) because glucose represses. There is still a basal rate of transcription. |
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Term
| What is the transcription rate of the lac operon when there is lactose present but no glucose. |
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Definition
| The transcription rate is at its highest because it is being induced by lactose and not being repressed by glucose |
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Term
| What is the transcription rate of the lac operon when glucose and lactose are present? |
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Definition
| The transcription rate is low because it is being repressed by glucose. This rate is low but higher than the basal rate because it is not fully repressed |
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Term
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Definition
| It is an anabolic operon that codes for the amino acid trpytophan. |
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Term
| How is the trp operon regulated? |
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Definition
There is a tryptophan repressor protein (RPtrp) which is inactive unless it is bound by the co-repressor tryptophan.
Tryptophan binds the repressor protein and activates it and the repressor binds to the operator, slowing down transcription.
So if the tryptophan is present the operon activity is reduced because the cell doesn't need as much tryptophan. |
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