Tryptophan Operon Attenuation What is the trp operon

Tryptophan Operon, Attenuation

What is the trp operon? • Escherichia coli need amino acids to survive—because, like us, they need to build proteins. • One of the amino acids they need is tryptophan. • If tryptophan is available in environment, E. coli will take it up and use it to build proteins. • E. coli can also make their own tryptophan using enzymes that are encoded by five genes. • These five genes are located next to each other in what is called the trp operon.

• The trp operon is a repressible system. • Repressible system - the effector molecule interacts with the repressor protein such that it can bind to the operator. • For the trp operon , the addition of tryptophan (the effector molecule) to the E. coli shuts off the transcription system because the repressors binds at the operator.

Structure of the trp operon • The trp operon includes five genes that encode enzymes needed for tryptophan biosynthesis, along with a promoter (RNA polymerase binding site) and an operator (binding site for a repressor protein). • genes of the trp operon are transcribed as a single m. RNA.

Turning the operon "on" and "off" • What does the operator do? • This stretch of DNA is recognized by a regulatory protein known as the trp repressor. • When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme.

High / Sufficient Tryptophan in cell • The trp repressor does not always bind to DNA. • When tryptophan is around, it attaches to the repressor molecules and changes their shape so they become active. • Instead, it binds and blocks transcription only when tryptophan is present.

Low/Insufficient Tryptophan • When there is little tryptophan in the cell, the trp repressor is inactive (because no tryptophan is available to bind to and activate it). • It does not attach to the DNA or doesnot block transcription, • and this allows the trp operon to be transcribed by RNA polymerase.

Attenuation of the trp Operon • One element of trp operon is leader sequence (L) that in immediately 5' of trp. E gene. • This sequence about 160 bp in size a controls expression of operon through a process called attentuation. • This sequence has four domains (domain 1, domain 2, domain 3 and domain 4). • Domain 3 (nucleotides 108 -121) of m. RNA can base pair with either domain 2 (nucleotides 74 -94) or domain 4 (nucleotides 126 -134). • If domain 3 pairs with domain 4, a stem and loop structure forms on m. RNA and transcription stops. • This structure forms when level of tryptophan is high in the cell.

Attenuation at low levels of tryptophan • If domain 3 pairs with domain 2, then stem and loop structure does not form and transcription continues through operon, and all of the enzymes required for tryptophan biosynthesis are produce. • These events occur when tryptophan is low in the cell.

• In general biological systems are quite error prone — in the case of the trp operon the repressor is reported to decrease expression by about 70 fold. • Attenuation can add another 10 fold decrease in expression. Both mechanisms (repressible system and attenuation system) thus work together to give a better level of control.