The tryptophan operon of E.coli.

Unlike lack operon tryptophan operon is an anabolic operon, which means that the structural genes are only transcribed when the amino acid tryptophan is absent from the source i.e. When the amino acid is available the structural genes are not synthesized. The enzymes are not required and hence the operon is shut off. The operon model was elucidated by Charles Yonofsky and his colleagues. The operon model consists of the following genes.
1) Structural genes - The structural genes are coded from A to the letter E and encoded for the enzymes required for tryptophan biosynthesis and the gene spans 6800basepairs.
TrpA - tryptophan synthase α
TrpB - tryptophan synthase β
TrpC - PRA Isomerase and InGP synthase
TrpD - Anthranilate synthase 2
TrpE - Anthranilate synthase 1.

2)The promoter and the operator region of 150 bp upstream to the structural gene TrpE.

3) Leader region of 162basepairs, situated between the operator and TrpE. And within the leader region the attenuator site is present. The leader region and its role is briefly explained below under regulation of operon by attenuation.

4) The Regulatory gene - TrpR upstream to the promoter. The promoter is 150bp in length

The Regulation of the Tryptophan operon.
1) Regulation by "Repression" - The Tryptophan operon is under the negative control just like the lac operon, but it is repressible whereas lac operon is inducible operon. By repressible i mean that the effector molecule can repress the operon. The operon is regulated by Repression by the transcription of the TrpR region i.e the Repressor gene which encodes for the "Aporepressor protein." However the Aporepressor protein is not an active Repressor itself, hence it binds with tryptophan to produce an active Repressor. This active Repressor binds to the operator thereby preventing the transcription of the structural genes by blocking the path for Rna polymerase. This phenomenon is known as "Repression" and it reduces Tryptophan biosynthesis by 70 fold hence reducing the cellular try levels.
In the absence of tryptophan the operon is turned on because no active repressor can be formed hence the operator is free for Rna polymerase to bind and carry on transcription of the structural genes.

Regulation of Tryptophan operon by "Attenuation" - To understand the attenuation mechanism we must first understand the details of the Leader region which consists of 4 regions - 1,2,3,4. And among these 1,2 2,3 and 3,4 are complementary. The region 1 consist the tryptophan coding sequence i.e. UGG UGG from 54-59 and region 4 consist of the stop coding sequence i.e. UGA from 69-71. The base pairing at 1 and 2 region causes pause signal, The base pairing at 3 and 4 region causes termination signal and the base pairing at 2 and 3 region causes the anti termination signal.
Mechanism - The model here is based on the fact that Transcription and Translation in prokaryotes is carried out as coupled process due to the lack of a membrane bound nucleus. Hence here the Ribosome position plays an important role in regulation which is ultimately decided by the Leader region. It can be explained as follows.
A) In the absence of Tryptophan - In the absence of Tryptophan the trpt.Rna.trp levels drops hence the Ribosome translating the mRna stalls at the Trp codons (54-49). Therefore the leader peptide is not completed. The Ribosome blocks the region 1, hence the region 2 and 3 base pairing occurs. Which is an anti termination signal therefore the Polymerase continues translating past the attenuator region and the structural genes. Therefore the Trp operon is turned on in the absence of Tryptophan.
B) In the presence of Tryptophan - In the presence of Tryptophan, sufficient trptRnatrp's are present, hence the Ribosome translate the mrna and finally stalls at the stop region on the Leader peptide. The stop codon is region 2 and (69-71) region. The covering of region 2 by Ribosome facilitates region 3 and 4 base pairing which encodes for the termination signal. Hence the Rna polymerase stalls there and the structural genes are not transcribed. The Trp operon is shut of In the presence of Tryptophan.

3) Regulation of Tryptophan operon by Feed back inhibition - If the amino acid Tryptophan accumulates at a faster rate than utilized then the feed back inhibition is triggered. The end product of the biosynthetic pathway recognize the 1st enzyme of the pathway and inacivates it by altering its confirmation hence the pathway is shut off. Once the conc falls the product is dissociated and the enabling pathway resumes.

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