In an operon for synthesis of an amino acid that is...

Name: Problem 14, Chapter 8: Textbook of Biochemistry: With Clinical Correlations
Uploaded: 2020-12-19T22:24:39-08:00
Duration: 4 min 1 s
Channel: Alyssa Mcalarney
Description: In an operon for synthesis of an amino acid that is controlled wholly or in part by attenuation, why does the presence of the amino acid prevent transcription of the whole operon while the absence of the amino acid permits it?

In an operon for synthesis of an amino acid that is controlled wholly or in part by attenuation, why does the presence of the amino acid prevent transcription of the whole operon while the absence of the amino acid permits it?

Key Concepts

Attenuation

Attenuation is a transcription regulation mechanism in prokaryotes where the formation of alternative RNA secondary structures in the leader region determines whether transcription is prematurely terminated or allowed to continue. This mechanism allows the cell to precisely control gene expression in response to changes in metabolite (e.g., amino acid) levels.

Coupling of Transcription and Translation

In bacteria, transcription and translation occur concurrently, meaning that the progress of the ribosome on the nascent mRNA influences the folding and subsequent structure of that mRNA. This close coupling is essential for regulatory mechanisms like attenuation, where the position and movement of the ribosome determine whether a terminator or antiterminator structure forms.

Leader Peptide and Ribosome Stalling

The leader peptide, encoded at the beginning of the operon, plays a pivotal role in attenuation by providing a sensing mechanism. When the amino acid is scarce, the ribosome stalls during translation of the leader peptide due to a shortage of charged tRNAs, which leads to the formation of an antiterminator structure, allowing full transcription. Conversely, when the amino acid is abundant, the ribosome translates the leader peptide quickly, permitting the formation of a terminator structure that causes premature transcription termination.

Transcript

00:01 So let's say we have an operon that's going to encode for enzymes to make an essential amino acid.

00:07 And this is going to be regulated similarly to the trip operon.

00:12 So in order to answer the rest of this question, we first have to remember how the trip operon works.

00:17 And we'll draw two examples right here of how it can work.

00:22 So in this first example, we're going to have increased tryptophan.

00:26 And in the second example, we're going to have decreased tryptophan.

00:30 So here we have a tryptophan repressor, and then here is the triptophan amino acid itself.

00:44 So when there's high amounts of triptophan, it is able to come in and bind to the repressor.

00:52 So right now, without triptophan bound to it, it's inactive.

00:56 And when tryptophan binds to it, it makes it active.

01:00 So we'll do the a with a circle just means active.

01:04 And when it's active, it's able to go to the dna and bind to it.

01:10 Now, this is actually going to prevent rna polymerase.

01:18 It's going to prevent rna polymerase from coming in and binding to the dna.

01:23 So if rna polymerase can't come in, then there's not going to be transcription of the enzymes needed to make tryptophan.

01:32 So no transcription.

01:38 Now if we have low tryptophan, and there's a triptophan repressor again, now we don't have the tryptophan to bind to the repressor and activate it, so it's going to remain inactive...

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