32. When tryptophan in excess most of times RNA polymerase dismount after transcription of first 150 nt in trp operon. This is termed as
(1) Anti-termination
(2) Attenuation
(3) Catabolite repression
(4) Feedback inhibition

Introduction

The trp operon in Escherichia coli is a classic model for understanding gene regulation in bacteria. It controls the biosynthesis of tryptophan, an essential amino acid, and employs multiple regulatory strategies to ensure efficient use of cellular resources. One of the most fascinating mechanisms is attenuation, which fine-tunes gene expression in response to intracellular tryptophan levels. When tryptophan is in excess, most RNA polymerase molecules terminate transcription prematurely after synthesizing only a short segment of the mRNA—about 150 nucleotides. This sophisticated process is called attenuation.

What Is Attenuation?

Attenuation is a regulatory mechanism that reduces the expression of the trp operon when tryptophan levels are high. Unlike repression, which blocks the initiation of transcription, attenuation prevents the completion of transcription by causing RNA polymerase to dismount early, resulting in a truncated mRNA that does not encode the biosynthetic enzymes for tryptophan1235678.

How Does Attenuation Work?

• The trp operon contains a leader sequence (trpL) upstream of the structural genes.

• This leader sequence includes an attenuator region that can form different RNA secondary structures (hairpins) depending on the availability of tryptophan-charged tRNA.

• When tryptophan is abundant, the ribosome quickly translates the leader peptide, allowing the formation of a terminator hairpin (regions 3 and 4 of the mRNA).

• This terminator hairpin causes RNA polymerase to dismount, terminating transcription after about 150 nucleotides, before the structural genes are transcribed2567.

• As a result, only a short, non-functional mRNA is produced, and the enzymes for tryptophan synthesis are not made.

Biological Significance

Attenuation allows E. coli to rapidly and efficiently shut down tryptophan biosynthesis when the amino acid is plentiful, providing a second layer of control beyond the trp repressor. This dual control mechanism ensures that the cell does not waste energy producing enzymes it does not need1245678.

Distinguishing Attenuation from Other Regulatory Mechanisms

• Anti-termination: This is a process where transcription is allowed to continue past a termination site, the opposite of attenuation.

• Catabolite repression: This refers to the inhibition of operons like the lac operon in the presence of a preferred carbon source (e.g., glucose), unrelated to the trp operon’s attenuation.

• Feedback inhibition: This is the direct inhibition of an enzyme’s activity by the end product, not a transcriptional regulatory mechanism.

Correct Answer

(2) Attenuation

When tryptophan is in excess, most RNA polymerase molecules terminate transcription after the first 150 nucleotides in the trp operon due to attenuation1256789.

Summary Table

Conclusion

Attenuation is a unique and elegant regulatory mechanism in the trp operon of E. coli. It ensures that, when tryptophan is abundant, transcription is prematurely terminated, preventing unnecessary synthesis of tryptophan biosynthetic enzymes. This process highlights the sophistication of bacterial gene regulation and the efficiency of cellular resource management.

Keywords: trp operon, attenuation, early termination, tryptophan, RNA polymerase, leader sequence, E. coli, gene regulation, transcriptional control, molecular biology

In summary:
Attenuation is the process by which excess tryptophan causes RNA polymerase to terminate transcription early in the trp operon.