Question

Trp Operon Examine the following trp operons. For each, explain whether you’d expect to see the trp operon transcribed when: 1) Tryptophan level is high, translation is occurring 2) Tryptophan level is low, translation is NOT occurring G – Genome + = Present/functional P – Plasmid - = Absent/non-functional O – Operator L(#) – Leader sequence with region (#) mutated P – Promoter R – Repressor Rs – Repressor that is unable to bind tryptophan Operon 1 G: O+, L(none), P+, R+

Operon 2 G:O+, L(1), P+, R+

Operon 3 G:O+, L(4), P+, R+  

Operon 4 G: O+, L(none), P+, R+

P: O⁺, L(3), P^-, R⁺       

Operon 5 G:O⁺, L(1), P⁺, R⁺                         

P: O⁺, L(4), P⁺, R^-

Answer 1

Trp operon is a repressible operon, which ,means in the presence of trp the operon is turned off. When trp is present it will go and bind to the repressor protein and activates it. Activated repressor will go and bind to the operator region of the promoter thereby preventing RNA polymerase from transcribing the trp operon ( since trp is present there is no need to synthesize more hence it is logical to shut down the operon). Another way to regulate trp operon is by a process called attenuation. Right after the promoter and the operator sequence and before the structural trp genes ( trp A to trp E genes) there is leader sequence with four different regions which can pair by complementary base pairing. Pairing of region 1 and 2 will pause the transcription, pairing of region 2 and 3 will allow the transcription and pairing of region 3 and 4 will terminate the transcription. In E coli the process of transcription and translation goes hand in hand. So when the transcription is occurring translation also starts along with it. Once translation starts region 1 and 2 pairs and pauses transcription ( which is occurring simultaneously) and when the ribosome reaches the leader sequence the region 1 has 2 trp codons. If trp amino acid is not present the ribosomes will be unable to incorporate trp in that region. This causes region 1 and 2 to unpair and instead 2 and 3 pairs up to allow transcription. But is the trp amino acid is present and ribosome was able to incorporate it when it was decoding the region 1 then the region 1 and 2 will unpair and region 3 and 4 will pair to cause the termination of trp operon transcription.

Operon 1 G: O+, L(none), P+, R+

Here leader sequence functional. So transcription control by attenuation is possible. promoter is functional and repressor is functional. So transcription control by repressor is also possible

1) Tryptophan level is high, translation is occurring -  

Transcription will not occur since there will be transcription control by repressor and by the process of attenuation

2) Tryptophan level is low, translation is NOT occurring

Transcription will be high since trp level is low hence repressor protein cannot be activated.

Operon 2 G:O+, L(1), P+, R+

1) Tryptophan level is high, translation is occurring -  

Here since region one is mutated in leader sequence control by attenuation is not possible. so there will be transcription control only by repressor. So transcription level will still be low but will still occur.

2) Tryptophan level is low, translation is NOT occurring

Transcription will be high since trp level is low hence repressor protein cannot be activated.

Operon 3 G:O+, L(4), P+, R+  

1) Tryptophan level is high, translation is occurring -  

Here since region 4 is mutated in leader sequence termination of transcription by attenuation is not possible. so there will be transcription control only by repressor. So transcription level will still be low but will still occur.

2) Tryptophan level is low, translation is NOT occurring

Transcription will be high since trp level is low hence repressor protein cannot be activated and control by leader sequence will not affect when trp level is low.

Operon 4 G: O+, L(none), P+, R+

P: O⁺, L(3), P^-, R⁺       

1) Tryptophan level is high, translation is occurring -  

Here the genome has function trp operon wil proper leader sequence promoter and operator. But the plasmid has no promoter, and the region three in leader sequence is mutated. Transcription will not be there since genome has all the functional regions of trp operon.

2) Tryptophan level is low, translation is NOT occurring

Transcription will be high since trp level is low hence repressor protein cannot be activated and control by leader sequence will not affect when trp level is low.

Operon 5 G:O⁺, L(1), P⁺, R⁺                         

P: O⁺, L(4), P⁺, R^-

1) Tryptophan level is high, translation is occurring -  

here leader sequence region 1 is mutated in the genome, but no in the plasmid. Hence control the attenuation is still possible since region one can be provided by the plasmid. Plasmid had no functional repressor but the genome has, so control by repressor is also possible. So transcription will not occur since control by repressor and attenuation process is still possible under these conditions.

2) Tryptophan level is low, translation is NOT occurring

Transcription will be high since trp level is low hence repressor protein cannot be activated and control by leader sequence will have no effect when trp level is low.