The Tryptophan Operon:
The 20 common amino amino acids are required in large amount for protein synthesis, and E.coli can synthesize all of them. The genes for the enzymes needed to synthesize a given amino acid are generally clustered in an operon and are expressed whenever existing supplies of that amino acid are inadequate for cellular requirements. When the amino acid is abundant the biosynthetic enzymes are not needed and the operon is repressed.
The E.coli tryptophan operon includes five genes for the enzymes required to convert chorismate to tryptophan. The mRNA from the tryptophan operon has a half life of only about three minute, allowing the cell to respond rapidly to changing needs for this amino acid.
Regulatory Sequence
This operon is regulated by two mechanisms:
- The repressor binds to its operator
- The transcription of tryptophan mRNA is attenuated.
The tryptophan repressor is a homodimer, each subunit containing 107 amino acid residues. When tryptophan is abundant it binds to the trp repressor, causing a conformational change that permits the repressor to bind to the tryptophan operator and inhibit expression of the tryptophan operon. The tryptophan operator site overlaps the promotor, so binding of the repressor blocks binding of the RNA polymerase.
Transcription Attenuation
It is a second regulatory process, in which transcription is initiated normally but is abruptly halted before the operon genes are transcribed. The tryptophan operon attenuation mechanism uses signals encoded in four sequences within a 162 nucleotide Leader region at the 5' end of the mRNA, preceding the initiation codon of the first gene. Within the leader lies a region known as attenuator made up of sequences 3 and 4.These sequences base pair to form a G=C rich stem and loop structure closely followed by a series of U residues. The attenuator structure act as a transcription terminator. If sequence 2 and3 base pair the attenuator structure cannot form and transcription continues into the tryptophan biosynthetic genes.
Mechanism:
When tryptophan concentrations are high, concentrations of charged tryptophan tRNA are also high. This allows translation to proceed rapidly past the two tryptophan codons of sequence 1 and into sequence 2, before sequence three is synthesized by RNA polymerase. In this situation, sequence 2 is covered by ribosome, and unavailable for pairing to sequence3 when sequence3 synthesized; the attenuator structure sequence3 and 4 forms and transcription halts.
When tryptophan concentration are low, the ribosome stalls at the two tryptophan codons in sequence 1 because the charged tryptophan tRNA is unavailable. Sequence 2 is synthesized, allowing these two sequences to base pair and permitting transcription to proceed.
