36. E.coli are growing in a medium with
glucose as the sole carbon source.
Tryptophan suddenly added. The cells
continue to grow, and divide every 30
minutes. What will be the status of
tryptophan synthase activity in the cells over
time if the trp mRNA remains stable
(1) Low
(3) No change
(2) High
(4) No activity

 

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The regulation of tryptophan biosynthesis in Escherichia coli is a classic example of how bacteria finely adjust their metabolism to changing environmental conditions. When E. coli is grown in a medium with glucose as the sole carbon source and tryptophan is suddenly added, several regulatory mechanisms come into play to control the expression and activity of the enzymes involved in tryptophan synthesis. This article examines the fate of tryptophan synthase activity under these conditions, especially if the trp mRNA remains stable, and what this means for bacterial metabolism.

Overview of Tryptophan Biosynthesis in E. coli

The trp operon in E. coli encodes a set of enzymes required for the biosynthesis of tryptophan, an essential amino acid. The last enzyme in this pathway is tryptophan synthase, which catalyzes the conversion of indole and serine into tryptophan. The expression of the trp operon is tightly regulated at multiple levels, including repression, attenuation, and feedback inhibition7.

Regulation of the trp Operon

When tryptophan is present in the environment or inside the cell, it acts as a corepressor, binding to the trp repressor protein. The repressor-tryptophan complex then binds to the operator region of the trp operon, preventing RNA polymerase from initiating transcription. This is known as repression.

Additionally, the trp operon is regulated by attenuation, a mechanism that allows the cell to terminate transcription prematurely when tryptophan is abundant. The leader sequence of the trp mRNA contains regulatory elements that form hairpin structures, leading to transcription termination if tryptophan is present.

Finally, tryptophan synthase and other enzymes in the pathway are subject to feedback inhibition, where tryptophan itself acts as an inhibitor of the enzyme activity.

Scenario: Glucose as the Sole Carbon Source and Sudden Addition of Tryptophan

Consider E. coli growing in a medium where glucose is the only carbon source. The cells are metabolically active, and most of the glucose is used for central metabolism, with some entering the pentose phosphate pathway1. When tryptophan is suddenly added to the medium, the cell senses the increased availability of this amino acid and responds by downregulating the trp operon.

What Happens to trp mRNA?

The question specifies that the trp mRNA remains stable. In normal conditions, when tryptophan is present, the trp mRNA is not synthesized at high levels due to repression and attenuation. However, if for some reason the trp mRNA is already present and stable (perhaps due to experimental manipulation or a mutation), it will persist in the cell.

What Happens to Tryptophan Synthase Activity?

Even if trp mRNA is stable, the presence of tryptophan will have several effects:

1. Repression of New mRNA Synthesis: The trp repressor will bind to the operator and prevent new transcription of the trp operon. However, since the trp mRNA is already present and stable, translation of the existing mRNA can continue.

2. Attenuation: In wild-type cells, attenuation would also reduce transcription, but this is not relevant if the mRNA is already present and stable.

3. Feedback Inhibition: Tryptophan synthase is subject to feedback inhibition by tryptophan. This means that even if the enzyme is present, its activity will be inhibited by the high concentration of tryptophan in the cell.

Implications for Enzyme Activity

Given that the trp mRNA is stable, translation can continue, and tryptophan synthase can be synthesized. However, the activity of the enzyme will be inhibited by the presence of tryptophan. This feedback inhibition ensures that the cell does not waste resources producing more tryptophan when it is already abundant.

In summary, even if tryptophan synthase is present in the cell, its activity will be low due to feedback inhibition by tryptophan. The cells will continue to grow and divide, as they have all the necessary nutrients, but the activity of tryptophan synthase will not be high.

Answering the Question

The question asks:
“E. coli are growing in a medium with glucose as the sole carbon source. Tryptophan suddenly added. The cells continue to grow, and divide every 30 minutes. What will be the status of tryptophan synthase activity in the cells over time if the trp mRNA remains stable?
(1) Low
(2) High
(3) No change
(4) No activity”

The correct answer is (1) Low. Even if trp mRNA is stable and tryptophan synthase is synthesized, the activity of the enzyme will be low due to feedback inhibition by tryptophan.

Biological Significance

The regulation of tryptophan synthase activity is crucial for metabolic efficiency. By inhibiting the enzyme when tryptophan is abundant, the cell avoids unnecessary energy expenditure on amino acid biosynthesis. This is especially important in environments where resources are limited or when the cell is growing rapidly.

Practical Applications

Understanding the regulation of tryptophan synthase activity is important for metabolic engineering and biotechnology. By manipulating the regulatory mechanisms, scientists can engineer E. coli strains that overproduce tryptophan for industrial applications24. For example, feedback inhibition can be alleviated by introducing mutations that make the enzyme less sensitive to tryptophan, allowing for higher production rates.

Glucose and Tryptophan Metabolism

Glucose is the preferred carbon source for E. coli, and its metabolism provides the precursors and energy required for amino acid biosynthesis1. When tryptophan is added to the medium, the cell can use it directly, bypassing the need for biosynthesis. This further reduces the demand for tryptophan synthase activity.

Stability of trp mRNA

If the trp mRNA is stable, it can continue to be translated, but the activity of the resulting enzyme will still be regulated by feedback inhibition. This highlights the importance of post-translational regulation in metabolic pathways.

Conclusion

When E. coli is grown in a medium with glucose as the sole carbon source and tryptophan is suddenly added, the activity of tryptophan synthase will be low, even if the trp mRNA remains stable. This is due to feedback inhibition by tryptophan, which ensures that the cell does not waste resources producing an amino acid that is already available. The cells will continue to grow and divide, but the activity of tryptophan synthase will be kept at a low level.

In summary, the correct answer is (1) Low. This regulatory mechanism is a key example of how bacteria efficiently control their metabolism in response to changing environmental conditions.