7.18B: The trp Operon - A Repressor Operon - Biology

7.18B: The trp Operon - A Repressor Operon - Biology

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Learning Objectives

  • Explain the relationship between structure and function of an operon and the ways in which repressors regulate gene expression

Bacteria such as E. coli need amino acids to survive. Tryptophan is one such amino acid that E. coli can ingest from the environment. E. coli can also synthesize tryptophan using enzymes that are encoded by five genes. These five genes are next to each other in what is called the tryptophan (trp) operon. If tryptophan is present in the environment, then E. coli does not need to synthesize it; the switch controlling the activation of the genes in the trp operon is turned off. However, when tryptophan availability is low, the switch controlling the operon is turned on, transcription is initiated, the genes are expressed, and tryptophan is synthesized.

A DNA sequence that codes for proteins is referred to as the coding region. The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon. Just before the coding region is the transcriptional start site. This is the region of DNA to which RNA polymerase binds to initiate transcription. The promoter sequence is upstream of the transcriptional start site. Each operon has a sequence within or near the promoter to which proteins (activators or repressors) can bind and regulate transcription.

A DNA sequence called the operator sequence is encoded between the promoter region and the first trp-coding gene. This operator contains the DNA code to which the repressor protein can bind. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes shape to bind to the trp operator. Binding of the tryptophan–repressor complex at the operator physically prevents the RNA polymerase from binding and transcribing the downstream genes.

When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized. Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators.

Key Points

  • The operator sequence is encoded between the promoter region and the first trp-coding gene.
  • The trp operon is repressed when tryptophan levels are high by binding the repressor protein to the operator sequence via a corepressor which blocks RNA polymerase from transcribing the trp-related genes.
  • The trp operon is activated when tryptophan levels are low by dissociation of the repressor protein to the operator sequence which allows RNA polymerase to transcribe the trp genes in the operon.

Key Terms

  • repressor: any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
  • operon: a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together

Trp Operon

The trp operon of E. coli contains five major structural genes encoding all seven protein functional domains necessary for tryptophan biosynthesis from the common aromatic precursor, chorismate. Transcription of the trp operon is highly regulated. Initiation of transcription at the trp promoter is regulated by the tryptophan-activated trp repressor. The repressor can bind at multiple operator sites located in the promoter region. Transcription of the structural genes of the trp operon also is regulated by transcription attenuation, in response to the accumulation of uncharged tRNA Trp . When this uncharged tRNA accumulates, it leads to ribosome stalling during attempted translation of the leader peptide coding region. This leads to the formation of an RNA antiterminator structure that prevents the RNA terminator structure from forming. Absence of the terminator structure allows polymerase to continue transcription into the structural genes of the operon. When there are adequate levels of charged tRNA Trp in the cell, the ribosome translating the leader peptide coding region completes leader peptide synthesis, allowing the RNA terminator structure to form, and terminate transcription.

7.18B: The trp Operon - A Repressor Operon - Biology

Lac Operon allows bacteria too break down lactose. However these genes are only active when lactose is present.

  • 3 Genes: Lac Z, Lac Y and Lac A
  • When expressed these 3 genes will to translated into proteins, which breakdown latose.

Bellow is an animation of Lac Operon,

add lactose when you are ready

NOTE: There is a step missing, for simplicity sake, I skipped the step of making the mRNA into proteins.

As you can see from the animation above, when no lactose is added, the Repressor molecule stays attached to the operator. Preventing RNA polymerase from attaching.

However, when Lactose is added it attaches to the Repressor, changing its shape. The repressor releases, allowing RNA polymerase to read the 3 Genes, Lac Z, Lac Y and Lac A.

The proteins produced breakdown the lactose until there is none left. Once there is no more lactose left the repressor re- attaches to the operator, preventing RNA polymerase from attaching and reading the genes.

This is an example of Positive control.

The majority of what we know about gene regulation, comes from bacteria. Before we start there are a few differences between Pro and Eukaryotic gene regulation systems. The main difference is that Prokaryotes, such as bacteria, have an Operator and Repressor .

An example of an organisms ability to turn off and turn on genes, is the Trp Operon in bacteria.

Trp Operon allows bacteria too produce tryptophan (an amino acid) when there is none present in its environment. Humans do not have the ability to synthesis Tryptophan. Thus it is an essential amino acid. Meaning we must obtain it from our diet.

Trp Operon varies from the Lac Operon in two ways.

  • First there are 5 genes, instead of 3
  • Trp A, Trp B, Trp C, Trp D and Trp E
  • Second Lac Operon is a positive control, while Trp Operon is a negative control .
  • Observe the animation bellow.

This is a section of DNA, which sits just after the promoter. Acting as an ON and OFF switch for gene transcription.

This molecule attaches to the operator setting in the OFF position.

While the repressor is attached to the Operator it prevents RNA polymerase from attaching.

Bacteria will have two types of controls. Positive and negative controls. They both require an Operator and Repressor, however react differently depending on their environment.

This is better explained by the animations bellow.

As you can see by the animation above, when tryptophan is present, the repressor is changed to fit on the operator, preventing RNA polymerase from attaching.

When tryptophan is in low supply, the repressor changes shape, detaching from the operator allowing RNA polymerase to read the gene. These genes synthesise tryptophan.

Chapter 19: Control of Gene Expression: It’s How You Play Your Cards That Counts Summary

Exam 3 – Control of Gene Expression
Leave the first rating
Enhancers are
A) proteins located adjacent to promoters
B) distant sites where regulatory proteins bind
C) expediters of RNA polymerase capture
D) proteins that bind with repressors, deactivating them
E) a bacterial form of promoters
B) distant sites where regulatory proteins bind
When tryptophan is present in the medium, the transcription of tryptophan producing genes in E. coli is stopped by a repressor binding to the
A) trp repressor
B) trp operon
C) trp promoter
D) trp operator
E) trp polymerase
D) trp operator

Created by
Key concepts:
Rna Polymerase Binds To The
The Environment
Structural Gene
Terms in this set (10)

Enhancers are
A) proteins located adjacent to promoters
B) distant sites where regulatory proteins bind
C) expediters of RNA polymerase capture
D) proteins that bind with repressors, deactivating them
E) a bacterial form of promoters
B) distant sites where regulatory proteins bind

When tryptophan is present in the medium, the transcription of tryptophan producing genes in E. coli is stopped by a repressor binding to the
A) trp repressor
B) trp operon
C) trp promoter
D) trp operator
E) trp polymerase
D) trp operator

When tryptophan is abundantly present in the environment of E. coli, the tryptophan binds to the
A) trp operon
B) trp promoter
C) trp operator
D) trp repressor
E) trp polymerase
D) trp repressor

In the function of the lac operon in E. coli, the lac genes are transcribed in the presence of lactose because
A) RNA polymerase binds to the operator
B) the repressor cannot bind to the promoter
C) an isomer of lactose binds to the repressor
D) CAP does not bind to the operator
E) of the absence of cAMP
C) an isomer of lactose binds to the repressor

The role of methylation of DNA is
A) up-regulating DNA transcription.
B) down-regulating DNA transcription.
C) prevention of mutation
D) irrelevant to gene transcription
B) down-regulating DNA transcription

E. coli is able to use foods other than glucose in the absence of available glucose, because falling levels of glucose cause an increase of
C) lactase
D) glu operons

In the absence of glucose, E. coli can import lactose to change into glucose and galactose because CAP binds to the
C) lac operon
D) operator
E) repressor

Which is not part of the lac operon?
A) repressor
B) activator protein
C) operator
D) promotor
E) structural gene
B) activator protein

In an operon the location of the regulatory region occurs __ the structural genes.
A) after
B) within
C) before
C) before

Proteins that block the passage of RNA polymerase are called:
A) operons
B) activators
C) repressors
D) enhancers
E) promoters
C) repressors

What are the 3 types of gene regulation in bacterial cells? At what location in the “central dogma” schematic do they act on?
transcriptional between DNA and mRNA, translational between mRNA and protein, and post translational between protein and activated protein.

Of the 3 types of regulation, which is the most energy efficient? (ie. spends the least amount of energy)
transcriptional because it stops the cell at the earliest stage.

Of the 3 types of regulation, which is the most efficient in terms of speed?
post translational because it is like an on/off switch and the protein is already made.

Complete the statement. Gene expression regulation allows organisms to respond to _ _ _
Changes in environment

E. coli can use a variety of sugars to make ATP. Which is most preferred and why?
Glucose–> starting compound for glycolysis

When would an E.coli switch to using Lactose instead of Glucose?
If Glucose is absent

What is an inducer?
something that triggers transcription of a certain gene

What is the inducer in E.Coli’s synthesis of the enzyme that breaks down Lactose?
Lactose itself

What is negative control/regulation?
when a regulatory protein called a repressor binds to DNA and shuts down transcription

What is positive control?
when a regulatory protein called an activator binds to DNA and triggers transcription

Are activators and repressors sequences of DNA or proteins?

What is a constitutive mutant?
A mutant cell that produces a certain product at all times (no regulation/response to environment)

What is an operon?
a unit of DNA containing a cluster of genes that are regulated by the same single regulatory factor.

What does the operon contain aside from the cluster of genes?
promoter, operator, and genes.

In an operon, do the different genes have different promoters?
NO, the gene cluster acts as one big gene with one single promoter

How does Lactose “turn on” the transcription of the enzyme to break lactose down? What does it bind to and what happens when it does?
binds to the repressor, which is attached to the DNA. Repressor then changes shape and falls off the DNA.

What part of the DNA is the repressor attached to?
the operator ,

What is an operator? (DNA, protein, etc)
sequence of DNA

What is inducer exclusion and how does Glucose act as an inducer excluder?
Inducer exclusion= when an inducer is prevented from activating a gene
Glucose prevents transport of Lactose to the repressor of the lac operon.

What is Global Gene Regulation?
the coordinated regulation of many genes

What is a regulon and how is it an example of global gene regulation?
A regulon is a group of different genes or operons that contain the same regulatory sequences and are controlled by the same type of regulatory protein…regulates many genes/operons at one time.

7.18B: The trp Operon - A Repressor Operon - Biology

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1) % of Reducing Sugar in:-
a) Unripe Banana = 0.5
b) Ripe Banana = 1.3
c) Overripe Banana = 3

2) % of Starch Content in:-
a) Unripe Banana = 25
b) Ripe Banana = 15
c) Overripe Banana = 4

3) % of Reducing Sugar in:-
a) Unripe Frozen Banana = 0.6
b) Unripe Refrigerated Banana = 0.9
c) Unripe Room Temperature Banana = 1.9
d) Unripe Banana Adjecent to Ripe Bananas at Room Temperature = 3.4

1) Why are the values of reducing sugars for the ripening of bananas stages(unripe, ripe, overripe) increasing while the starch content of each is decreasing?

2) How do temperatures affect the value of reducing sugar of unripe bananas? and why placing ripe bananas adjacent to unripe bananas have the observed effect on the banana ripening?

Assume the fire loss and lack of reinvestment of the proceeds resulted in the actual contraction of the business. Discuss whether the transaction would qualify as a partial liquidation and the reasons it would or would not qualify

Vectors A and B have equal magnitudes of 44.0. If the sum of A and B is the vector 10.5 j (head) , determine the angle between A and B .( i couldnt write the vectors signs to top of the A and B but they are vectors )

Part 1: Which statement is not associated with covalent catalysis by enzymes?

A) It never involves coenzymes B) A transient covalent bond is formed between the enzyme and the substrate

C) When the reaction is complete, the enzyme returns to iits origional state D) A new pathway from substrate(s) to product(s) is formed that is faster than the uncatalyzed reaction.

Part 2: Which statement is false for a competitive inhibitor?

A) It does NOT change the Vmax B) a'= 1.0 C) it is irreversible D) It is often structurally similar to substrate

Part 3: Which statement about enzymes is FALSE?

A) A substantial amount of their catalytic power results from binding of the substrate(s) through weak interactions. B) They lower the activation energy of a reaction. C) They alter the overall thermodynamics of a reaction. D)They increase the rates of reactions by 10^5-fold to 10^17-fold.

Describe in detail an experiment to test whether RNA is required for protein synthesis. Underline the following terms in your answer: cell lysate, ribonuclease, in vitro, and protein purification. Draw one figure supporting your answer

Please help me with identify which represent application ofrecombinant DNA technology or it it can be use in an application.If it does not represent an application write NO

Also can you please give a little discription why

When a firm produces a level of output on the production function

a. Marginal physical product is zero.

b. Maximum efficiency is achieved.

c. Opportunity cost for resources is at a maximum.

Azaserine is a structural analog of glutamine. It is a competitive inhibitor of many enzymes that use glutamine as substrates. What three biosynthetic products would you expect to be inhibited by azaserine. Do you think that eating azaserine would be immediately fatal? Why or why not?

5’ untranslated region

The 5′ UTR is a frequently overlooked region for gene expression engineering. While several established tools enable the rational design of 5′ UTRs, such as RBS Calculator (Salis et al., 2009 ) and UTR designer (Seo et al., 2013 ), a substantial number of studies either fail to consider the 5′ UTR as a unique functional region, or simply consider it as a part of the promoter region (Decoene et al., 2018 Nijs et al., 2020 ). In this review, we want to highlight the importance of the 5′ UTR since it influences both transcriptional and translational processes in bacteria. During translation initiation, ribosomes bind to mRNA and build the translation machinery together with several accessory proteins. Ribosomes can bind to the Shine–Dalgarno (SD) sequence present within the 5′ UTR part of an mRNA through complementary base pairing of the 16S rRNA (Fig. 1). There are many studies in the literature that erroneously refer to the entire 5′ UTR as ribosome binding site (RBS), a term that is synonymous with the SD sequence as specified in the Sequence Ontology ( The MISO Sequence Ontology Browser ). The binding rate and strength of the translation machinery to the 5′ UTR affect how often a transcript will be translated. Translation rates are affected by the 5′ UTR sequence composition and properties, which can be modified to modulate gene expression as we will outline in the following subsection.

The role of 5′ UTRs in transcript and translation

Several studies point out the importance of the 5′ UTR sequence composition on transcript abundance and translation rates. Studies by Berg et al. ( 2009 ) and Lou et al. ( 2012 ) indicate that point mutations within the 5′ UTR not only lead to changes in translation rates but also affect transcript abundance, potentially through the creation of a more stable transcript. Moreover, Le et al. ( 2020 ) recently reported a novel dual UTR concept benefiting from the 5′ UTRs diverse role both in transcriptional and in translational processes. In their study, the mutagenesis of a 5′ UTR in E. coli resulted in 5′ UTR variants that could be classified into two groups based on their distinct expression profiles: 5′ UTRs leading to increased translation (Tn group) and 5′ UTRs leading to increased transcription of the reporter gene (Tr group). Single 5′ UTRs from the two groups were then combined into a dual UTR, composed of two 5′ UTRs and a spacer DNA sequence in between them. The dual UTR leads to the discovery of a synergistic effect that results in increased expression of the reporter gene. The group hence demonstrated that dual UTRs can be used to control both transcription and translation of the reporter gene. They also report that it is crucial to place the 5′ UTR of the two different groups in the correct order to benefit from the observed synergistic effect as the expression of the reporter gene was decreased when the order of the 5′ UTRs in the dual UTR was Tn-Tr. These synergy and anti-synergy effects highlight the two distinct roles that 5′ UTRs play in bacterial gene expression, influencing translational and transcriptional processes. It also highlights how RES composition and architecture can influence gene expression.

The 5′ UTR contains an initially transcribed region (ITS, Fig. 1), which influences promoter escape of RNAp (Heyduk and Heyduk, 2018 ). To study how different nucleotides in distinct positions affect RNAp promoter escape, Heyduk and Heyduk created a library of ITS variants, which covered up to a 144-fold difference in promoter escape kinetics. The first 10 nucleotides of the ITS influenced gene expression the most, requiring an A in position +1. As a follow-up experiment, the first 10 nucleotides of the ITS of different promoters were replaced with a fully randomized artificial sequence. Replacing the first 10 nucleotides of the ITS with every possible nucleotide combination, in their set-up, revealed that position +2 of the ITS requires a specific promoter-dependent nucleotide. Furthermore, nucleotide pairs of GG and GA within the ITS increased escape velocity, and high T content correlated with slow escape, specifically in the combination of (T/C)G. Similar effects were also seen in a study looking at the effect of ITS nucleotide composition at a genome-wide scale. Promoter escape seems to be hampered by T-rich ITS nucleotide sequences that can induce abortive transcription (a non-productive reiterative transcription process that occurs at most promoters leading to short transcripts in a template-dependent manner) (Imashimizu et al., 2020 ).

Another target for gene expression optimization is the junction between the promoter and 5′ UTR, and 5′ UTR and CDS. Mutalik et al. ( 2013a ) report that these junctions affect gene expression and that they can be optimized and used to regulate gene expression levels. The importance of junction optimization for gene expression has also been supported through a study by Mirzadeh et al. ( 2015 ). The group used degenerate primers to randomize the six nucleotides upstream of the start codon and exchange codons 2 and 3 with all possible synonymous codons. Changing the nucleotides at the 5′ UTR and CDS subsequently led to differences in gene expression levels by several orders of magnitude.

The above-mentioned findings indicate that several promoter structures/functions overlap within the 5′ UTR (Mutalik et al., 2013b ) (Fig. 1), even though some findings may stem from incorrectly annotated promoters and 5′ UTRs (Lou et al., 2012 ). Overall, controlling and studying gene expression through the 5′ UTR region presents a great potential (Ameruoso et al., 2019 ) as will be highlighted in the next subsection.

Modulating gene expression through the 5′ UTRs

Methods targeting the 5′ UTR can be used to vary gene expression by: mixing two or more different 5′ UTRs making hybrids of sequences that ribosomes can bind to (Isaacs et al., 2004 ) mutating the 5′ UTR (Huang et al., 2006 ) or replacing parts of conserved motifs or spacer regions with (semi-)artificial sequences (Min et al., 1988 Zhelyabovskaya et al., 2004 Zhang et al., 2015 Bonde et al., 2016 Oesterle et al., 2017 Shi et al., 2018 ) (Fig. 2). For example, in a study by Huang et al. ( 2006 ), protein production levels of E. coli alkaline phosphatase were improved by randomly mutating the 5′ UTR. Alkaline phosphatase activity was increased as much as sevenfold compared with the levels reached with the wild-type 5′ UTR. The mutations were studied separately, and it was concluded that the SD sequence had been affected by the mutagenesis step, creating a seemingly stronger motif. Notably, when the group studied the individual mutations, they found that the additive effects of single mutations were not enough to explain the sevenfold increase, suggesting a more intricate regulation of gene expression through the 5′ UTR than a simple addition of factors.

When modulating gene expression of metabolic pathways, there is a need to control the expression of multiple genes, for instance, in operons. In the assembly of pathways, the hybrid method is often used. For example, Pfleger et al. ( 2006 ) used the hybrid method to assemble a metabolic pathway, balancing gene expression by controlling the intergenic regions in an operon through the use of hairpins. The hairpins originated from a library of tuneable intergenic regions (TIGRs), which were inserted into the intergenic region of the operon. These intergenic hairpins post-transcriptionally controlled the expression of different genes in the operon, showing that hybrid 5′ UTRs can be used to tune the expression from a metabolic pathway. An interesting take on pathway assembly is the so-called Golden Mutagenesis (Püllmann et al., 2019 ). This pathway assembly approach combines hybrid pathway assembly with mutagenesis, thus combining two different randomizing methods into one.

Hybrid, mutagenized and (semi-)artificial 5′ UTRs can be used to study the fundamental biology of gene expression

As outlined above, methods that use random approaches can be used to modulate gene expression for protein production purposes. In addition, these methods are also valuable for studying the fundamental mechanistic of gene expression regulation. For example, in 1985 Whitehorn et al. ( 1985 ) used the hybrid method to show that the spacer length between an SD sequence and the start codon affects gene expression levels of the human β-interferon in E. coli. The group also found that single nucleotide changes in the spacer region had dramatic effects, increasing accumulation of human β-interferon to about 15% of the total cell mass. Holmqvist et al. ( 2013 ) studied how mutations in the 5′ UTR of the csgG gene would affect cis- and trans-regulation of the gene. The csgG mRNA is regulated by at least four small RNAs (sRNA) that repress translation by binding to specific stem–loop areas of the 5′ UTR. Introducing random mutations into the stem–loop area prevented binding of the sRNA and therefore led to increased expression, showing that random mutagenesis can also be used to regulate trans-effects.

Rational design approaches commonly rely on the use of known conserved motifs to introduce changes in nucleotide sequences. However, conserved motifs are not always necessary to drive gene expression. For example, the SD sequence was long thought to be necessary for ribosome binding to mRNA in all prokaryotes because the motif was found to be highly present when it was first described in E. coli (Shine and Dalgarno, 1974 ). The 16S rRNA ribosome subunit contains a conserved anti-SD sequence at its 3′-end that is thought to be essential for ribosome recruitment to the mRNA based on the complementary binding of these two sequences (Steitz and Jakes, 1975 ), supporting the notion of SD sequence-dependent translation. However, the SD sequence has since then been shown to be only partially conserved in other bacterial species, some species simply lacking an SD sequence for up to 88% of their genes (Chang et al., 2006 Nakagawa et al., 2017 ). Different organisms may also express genes from the so-called leaderless mRNAs lacking the 5′ UTR entirely (Zheng et al., 2011 ). A study by Fargo et al. ( 1998 ) reports that several genes could successfully be expressed both in E. coli and in the chloroplast of Chlamydomonas reinhardtii (a single-celled green alga) after SD sequences were removed, indicating the existence of SD-independent translational mechanisms. Another study found that ribosomes with a defect 16S-anti-SD sequence could still bind to start codons (Saito et al., 2020 ). Furthermore, strong SD motifs may fail to outcompete weaker SD motifs by binding to ribosomes strongly and therefore leading to translational arrest (Komarova et al., 2005 ).

The mRNA secondary structure around the start codon plays a crucial role in translational regulation (Kudla et al., 2009 Chiaruttini and Guillier, 2020 ). Translation initiation is considered to be a rate-limiting step of translation (Hersch et al., 2014 ) therefore, when ribosomes are prevented from binding to mRNA, it affects translation rates significantly (Duval et al., 2015 Gualerzi and Pon, 2015 ). Ribosomes can be prevented from binding to mRNA through strong mRNA secondary structures. It has been shown that mRNA building weaker secondary structures correlate with higher translation rates (de Smit and van Duin, 1994a ). Weak secondary structures occur more frequently in mRNA derived from A/T-rich regions in the genome (Kudla et al., 2009 Allert et al., 2010 ), while GC-rich mRNAs may build more stable secondary structures. Thus, 5′ UTRs can possibly be engineered to build weak secondary structures around the start codon to increase translation rates. However, strong GC-rich secondary structures only seem to prevent ribosomes from binding to the mRNA when no SD motif is present (Sterk et al., 2018 ). It is possible that accessible SD motifs counterbalance mRNA secondary structures that would otherwise prevent ribosome binding (de Smit and van Duin, 1994b Ma et al., 2002 ). Additionally, unstructured regions at the 5′-end of the mRNA can facilitate ribosome binding. Ribosomes can be loaded onto mRNA through non-specific binding of the 30S subunit to a ‘standby’ site. The ribosome can then relocate from the standby site onto the start codon (de Smit and van Duin, 2003 Sterk et al., 2018 ).

While strong secondary mRNA structures lead to increased mRNA half-life, weak secondary structures lead to short mRNA half-life (Bervoets and Charlier, 2019 ). It is somewhat contradictory that weaker and therefore short-lived mRNA structures have been shown to correlate with high translation levels. Hence, how translation is regulated at the mRNA level is yet to be fully understood. Overall, many different factors influence translation levels, such as the presence of specific sequences or nucleotides in various positions (context dependency) (Wu et al., 2018 ), the spacing between the motifs and the strength of mRNA secondary structures (Mortimer et al., 2014 Chiaruttini and Guillier, 2020 ).

Rills, gullies, streams, rivers and tributaries are all caused by

Water erosion moves water downhill and during this process, different landforms are formed. Some of them are rills, gullies, streams, rivers, tributaries, waterfalls, floodplains, meander, lakes, etc.

Because they all deal with some form of water

Explanation:All these bodies of water are formed when water flows from the top, such as from a hill, mountain, or down a valley, etc. In these high areas, melting snow causes tiny streams of water to flow down. They are also formed when small bodies of water combine to form larger bodies. For instance, A stream is a channel along which water is continually flowing down a slope. Unlike gullies, streams rarely dry up. Small streams are also called creeks or brooks. As streams flow together, they form larger and larger bodies of flowing water. A large stream is often called a river

D) trp repressor

Feedback: In the bacterium E. Coli, a group of five genes code for enzymes required to synthesise the amino acid tryptophan. All five genes are transcribed together as a unit called an operon.

An operon is a group of genes that is under the control of a single operator site. A regulatory protein called a repressor can bind to the operator site and prevent transcription. When typrophan is lacking in the environment, the repressor is inactive.

RNA polymerase binds to the promoter site and then proceeds down the DNA, transcribing the genes for the tryptophan biosynthesis enzymes.

When tryptophan is present in the environment, the organism no longer needs to make tryptophan. Tryptophan binds to the repressor and activates it.

The activated repressor now binds to the operator, located withing the tryptophan promoter and blocks transcription.

The tryptophan repressor is a helix-turn-helix regulatory protein. When tryptophan is absent from the environment, the repressor is in an inactive conformation and cannot bind to the DNA to prevent transcription.

When tryptophan is abundant, two molecules of tryptophan bind to the repressor.

This alters the orientation of the helix-turn-helix motifs in the repressor and causes their recognition helices to fit into adjacent major grooves of the DNA.

Thus the synthesis of tryptophan occurs when it is needed, but is repressed when tryptophan is available.