Tryptophan operon in E. coli

Tryptophan operon in E. coli:

Unlike lac operon, which is actually responsible for the control of the production of catabolic enzymes, tryptophan operon is responsible for controlling the production of biosynthetic enzymes. When the amino acid is abundant in the cell the biosynthetic enzymes are not necessary and the operon system is repressed in that case.

Like lac operon, tryptophan operon in E. coli also consists of two main portions i.e. regulatory portion and structural portion.

1.Regulatory portion:

Regulatory portion consists of repressor gene, promoter site, operator site and leader sequence.

2.Structural portion:

Structural portion consists of the trpE gene, trpD gene, trpC gene, trpB gene and trpA gene.

There are two mechanisms for regulation of tryptophan operon.

A.Simple switch ON/OFF regulation mechanism:

Case1: When tryptophan is abundant in the cell.

When tryptophan is abundant in the cell, it binds to the trp repressor protein synthesized by trp R gene causing a conformational change in trp. Repressor protein that permits repressor to bind to the operator site of the DNA. The binding of repressor protein to operator site blocks binding of RNA polymerase enzyme to its promoter site and hence no transcription of the structural genes takes place. So no enzymes will be produced in this case that will convert chorismate to tryptophan, i.e. tryptophan operon remains repressed in the presence of tryptophan in the cell.

Case2: When tryptophan is less abundant to cell

When tryptophan is less abundant or absent in cell, the tryptophan is not available for binding with repressor protein. Hence the conformation of repressor protein is not changed in this case. The inactive repressor protein synthesized by trp-R gene can no longer blind to the operator site and can't block the promotor site. RNA polymerase hence can bind the promotor site and can initiate transcription of all the structural genes.The polycistronic M RNA, which has coding for all fire enemies is translated into five polypeptides or enzymes that can convert chorismate into tryptophan.

B. Fine regulation mechanism : Attenuation

The simple ON/OFF mechanism by a repressor is not entire regulatory mechanism in operon. The different cellular concentration of tryptophan operon can vary the value of synthesis of biosynthesis enzymes. Once the repression is lifted the rate of transcription is fine regulated by a second regulatory process called transcription is attenuation in which the transcription is initiated normally but is abruptly halted before the operon genes are transcripted. This is possible bacteria because of very close coupling of transcription and translation in bacteria, i.e transcription and translation go side by side in bacteria. Transcription attenuation process is also known to be a process where transcription is controlled by translation. The tryptophan attenuation process uses signals encoded in four sequences within 162 nucleotides leader region at the 5' ends of M RNA preceding the initiated codon of the first structural genes. sequence1 in the leader region encodes for fourteen amino acids, two of which (10th and 11th) are for tryptophan amino acids. sequence 2,3 and four are in a such way that sequence 3 can base pair either with sequence 2 or sequence 4 in the m RNA . If sequences forms base pair with sequence 2, It does notform a stable hairpin like structure, whereas cohen sequence 3 base pairs with sequence 4, then it forms a stable hair pin like structure . This stable stem-lropstructure or hair pin like structure is called attenuater structure.

The leader peptide has no other known cellular functions than a regulatory device. The peptide is translated immediately after it is transcribed by a ribosome that follows closely behind RNA polymerase.

Case1: When there is abundant tryptophan in cell