14.1 Trp Operon

Question 1

What is the Trp Operon an example of?

Answer 1

Gene regulation through attenuation

Question 2

What is tryptophan?

Answer 2

• An amino acid essential for protein synthesis
• in e coli, enzymes that synthesize tryptophan are found in the Trp operon, can also be found in the environment

Question 3

Why is the trp operon regulated?

Answer 3

• because it can be synthesized by enzymes in e coli
• or can be found in the environment

Question 4

What are the two ways that expression of the trp operon is regulated?

Answer 4

• negative regulation (using a co-repressor)
• attenuation

Question 5

Describe the genes that are a part of the trp operon

Answer 5

• 5 genes (trp E through A)
• the genes encode enzymes needed for tryptophan biosynthesis
• once translated into proteins, the proteins are part of the complexes required to convert precurser (charismate) into tryptophan amino acid

Question 6

What are the units of the trp operon

Answer 6

• trpR(not a part of operon lol, located elsewhere on the chromosome)
• P, O, trpL (trp leader peptide) and then trp E through A

Question 7

Where does the trp repressor bind?

Answer 7

to the operator sequence

Question 8

Is the trp leader peptide transcribed onto the mRNA?

Answer 8

yes

Question 9

What happens when tryptophan binds to the trp repressor?

Answer 9

• the trp goes from inactive state to active state, and can now bind to the operator sequence

Question 10

What happens when the trp repressor binds to the operator sequence?

Answer 10

• RNA polymerase can bind to the promoter, but cannot push past the repressor
• lowered transcription occurs (75%)

Question 11

What type of regulation occurs when tryptophan binds to the trp repressor?

Answer 11

negative regulation, binding leads to decreased transcription

Question 12

What role does tryptophan play when it bidns to the trp repressor?

Answer 12

co-repressor

Question 13

Relationship between tryptophan and transcription

Answer 13

• high tryptophan = low transcription
• low tryptophan = high transcription

Question 14

What is the regulatory process of attenuation?

Answer 14

To lower the expression of genes

Question 15

What accomplishes attenuation in the trp operon?

Answer 15

the trp leader peptide, which contains 4 regions that have complementary sequences to each other

Question 16

What is called when there is stem loop structures between 1+2, and 3+4?

Answer 16

Attenuation sequence

Question 19

What is it called when there is a stem loop structure formed between regions 2 and 3 of the trp leader peptide sequence mRNA?

Answer 19

antitermination sequence

Question 19

What is important about region one of the trp leader peptide?

Answer 19

• 2 tryptophan codons
• t rna’s charged with tryptophan are required to produce the leader peptide

Question 20

When tryptophan levels are high

Answer 20

• a ribosome will begin initiating protein synthesis off of the rna strand that’s newly emerging from the RNA polymerase
• because there is plenty of tryptophan in the cell, there are plenty of charged “W” tRNAs that are ready to engage in protein synthesis
• the ribosome is able to produce the polypeptide from the leader peptide sequence, including the tryptophan residues that are encoded by the UGGUGG trp codons located in the first region of the trpL
• as the ribosome proceeds down the messenger RNA, it covers the first and second region of the leader peptide
• the polymerase is a little bit ahead, and has transcribed regions 3 and 4 that can form a stem loop structure while the ribosome is initiating protein translation
• the stem loop structure of 3 and 4 is followed by a series of UUU residues
• this stem loop poly U sequence forms a transcription termination sequence, allowing the RNA pol to fall off the transcript, and preventing transcription of anything downstream
• This stem loop structure is rho independent