The Tryptophan Operon – A Repressible Operon System
The tryptophan (trp) operon system is a type of repressible operon system. It was worked out by Jacob and Monod in 1953.
The 20 amino acids are required in large amounts for protein synthesis and E.coli can synthesize all of them. The genes for the enzymes needed to synthesize tryptophan are generally clustered in trp operon and are expressed whenever existing supplies are limiting.
When tryptophan is present, it binds the trp repressor and induces a conformational change in that protein, enabling it to bind the trp operator and prevent transcription (operon is repressed).
The E.coli trp operon includes five trp genes (trp E, D, C, B, A) that encode enzymes required to convert chorismate to tryptophan.
Transcription is initiated at the beginning of the 162 nucleotide mRNA leader encoded by a DNA region called trpL. Once repression is lifted and transcription begins, the rate of transcription is controlled by a second regulatory process, called transcription attenuation. This regulatory process determines whether transcription is attenuated (terminated) at the end of the leader or continues into the structural genes.
The trp operon attenuation mechanism uses signals encoded in four sequences within a 162 nucleotide leader region at the 5’-end of the mRNA, before the initiation codon of the first gene (trpE). Within the leader lies a region known as the attenuator, made up of sequences 3 and 4. The attenuator structure forms by the pairing of sequences 3 and 4. The attenuator structure acts as a transcription terminator.
Sequence 2 is an alternate complement for sequence 3. If sequences 2 and 3 base-pair, the attenuator structure cannot form and transcription continues into the trp genes. The 2:3 structure, unlike the 3:4 attenuator, does not prevent transcription.
The sequence encoding the leader peptide has two tryptophan codons in a row. When tryptophan concentrations are high, concentrations of charged trp tRNA are also high. This allow ribosome to quickly translates sequence 1 and block sequence 2. Ribosome blocking sequence 2 allows formation of the 3:4 attenuator, aborting transcription at the end of the leader RNA. The leader peptide has no other known cellular function, its synthesis is simply an operon regulatory device.
When tryptophan levels are low, there is very little charged tryptophan tRNA available, and the ribosome stuck when it reaches the tryptophan codons. A ribosome caught at the tryptophan codons, masks region 1, leaving sequence 2 free to pair with sequence 3, thus the 3:4 attenuator hair-pin structure cannot be made. In this way, RNA polymerase passes the attenuator and moves on into the operon, allowing trp enzymes expression.
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