Question

Explain how attenuation regulates the trp operon in response to cellular levels of tryptophan

Answer 1

Trp Operon

The trp operon—a group of genes that is used, or transcribed, together—that codes for the components for production of tryptophan. The trp operon is present in many bacteria, but was first characterized in Escherichia coli.

Trp operon contains five structural genes: trpE, trpD, trpC, trpB, and trpA, which encode enzymatic parts of the pathway. It also contains a repressive regulator gene called trpR. trpR has a promoter where RNA polymerase binds and synthesizes mRNA for a regulatory protein.

Trp Operon Structure

Attenuation of the trp Operon

• · One element of the trp operon is the leader sequence (L) that in immediately 5' of the trpE gene. This sequence about 160 bp is size also controls the expression of the operon through a process called attentuation.
• · This sequence has four domains (1-4). Domain 3 (nucleotides 108-121) of the mRNA 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 the mRNA and transcription stops. This structure forms when the level of tryptophan is high in the cell. If domain 3 pairs with domain 2, then the stem and loop structure does not form and transcription continues through the operon, and all of the enzymes required for tryptophan biosynthesis are produce. These events occur when tryptophan is low in the cell.
• · If domain 4 is deleted, the stem and loop structure can not form and transcription of the remainder of the operon will occur even in the presence of tryptophan. Domain 4 is called the attenuator because its presence is required to reduce (attenuate) mRNA transcription in the presence of high levels of tryptophan.
• · Domain 1 is also an important component of the attenuation process. The section of the leader sequence encodes a 14 amino acid peptide that has two tryptophan residues.

How does this entire attentuation process work? We will discuss the molecular events that occur under conditions of high and low tryptophan.

trp Operon Transcription Under High Levels of Tryptophan

When the cellular levels of tryptophan are high, the levels of the tryptophan tRNA are also high. Immediately after transcription, the mRNA moves quickly through the ribosome complex and the small peptide is translated. Translation is quick because of the high levels of tryptophan tRNA. Because of the quick translation, domain 2 becomes associated with the ribosome complex. Then domain 3 binds with domain 4, and transcription is attenuated because of the stem and loop formation.

trp Operon Transcription Under Low Levels of Tryptophan

Under low cellular levels of tryptophan, the translation of the short peptide on domain 1 is slow. Because of the slow translation, domain 2 does not become associated with the ribosome. Rather domain 2 associates with domain 3. This structure permits the continued transcription of the operon. Then the trpE-A genes are translated, and the biosynthesis of tryptophan occurs.

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