Question

Effect of mRNA and Protein Stability on Regulation \(E\). coli cells are growing in a medium with glucose as the sole carbon source. After the sudden addition of tryptophan, the cells continue to grow and divide every \(30 \mathrm{~min}\). Describe (qualitatively) how the amount of tryptophan synthase activity in the cells changes with time under each condition: a. The trp mRNA is stable (degrades slowly over many hours). b. The \(\operatorname{trp}\) mRNA degrades rapidly, but tryptophan synthase is stable. c. The \(\operatorname{trp}\) mRNA and tryptophan synthase both degrade rapidly.

Short answer

a. Stable enzyme levels increase; b. Enzymes increase then stabilize; c. Enzymes increase slightly, then decrease.

Step-by-step solution

Understanding mRNA Stability

When trp mRNA is stable, it means that even after it is produced, it remains in the cell for an extended period. This allows continuous production of the protein, tryptophan synthase, as long as translation machinery is available. Consequently, enzymes will accumulate as they are synthesized continuously over the hours without degradation of mRNA.

Rapid mRNA Degradation and Protein Stability

If trp mRNA degrades quickly but the resulting protein, tryptophan synthase, is stable, the cells can only produce new enzymes soon after mRNA synthesis occurs. However, due to protein stability, already synthesized tryptophan synthase enzymes remain active for longer durations. The enzyme levels initially rise but stabilize and persist over time.

Rapid Degradation of Both mRNA and Protein

When both trp mRNA and tryptophan synthase degrade swiftly, the synthesis of new proteins relies heavily on continuous mRNA transcription. Enzymes produced will not accumulate significantly, as both mRNA and protein deplete rapidly. Therefore, enzyme levels initially increase after mRNA synthesis but quickly decrease, leading to lower net activity over time.

Key concepts

mRNA Stability

The stability of mRNA refers to how long the mRNA molecules persist in the cell after being synthesized. mRNA stability is crucial in determining how much protein is synthesized from a gene.
For instance, when the trp mRNA in Escherichia coli is stable and degrades slowly, the mRNA exists in the cell for a prolonged period. This allows the ribosomes to translate it into tryptophan synthase enzymes continuously.
As a result, even with fluctuations in mRNA synthesis, stable mRNA ensures a consistent enzyme supply. This is because the mRNA is available for translation over an extended time, leading to sustained enzyme levels in the cells.
In contrast, unstable mRNA would degrade quickly, reducing its availability for translation, thereby impacting enzyme production negatively.

Protein Stability

Protein stability refers to how long a protein remains intact and functional after it is synthesized. It is a key factor in enzyme regulation and function within the cell.
In the scenario where tryptophan synthase, a specific protein in E. coli, is stable, it continues to perform its function for an extended period even if the mRNA that codes for it has degraded.
This stability helps maintain enzyme activity levels, as the proteins do not degrade as quickly as they are produced.

• Proteins with high stability are not rapidly degraded, which allows them to accumulate in the cell.
• This ensures prolonged activity even if new synthesis reduces after the initial surge due to mRNA degradation issues.

Therefore, protein stability is a critical aspect in regulating the amount and activity of proteins like tryptophan synthase in the cell.

Enzyme Synthesis

Enzyme synthesis is the process through which cells produce enzymes by translating mRNA messages. This process is heavily influenced by both mRNA and protein stability.
In E. coli, after the sudden addition of tryptophan, the synthesis of tryptophan synthase depends on continuous mRNA transcription if both mRNA and protein degrade rapidly.
When enzyme synthesis is efficient, the cell manages to produce enough enzymes to meet its metabolic needs, provided that the mRNA templates and stable proteins are available.
However, rapid degradation of mRNA or proteins undermines this synthesis, making it challenging to maintain adequate enzyme levels. Therefore, the stability of these molecules is crucial to provide a steady supply of enzymes, ensuring cellular functions are not disrupted.

Escherichia coli

Escherichia coli (E. coli) is a widely studied bacterium that serves as a model organism in molecular biology. This bacterium adapts quickly to changes in its environment, such as nutrient availability.
In the presence of glucose and the sudden addition of tryptophan, E. coli adjusts its gene expression to produce or reduce enzymes needed for its biosynthesis pathways.

• E. coli utilizes glucose efficiently, but when tryptophan is added, it responds by potentially down-regulating its own tryptophan biosynthesis pathways.
• This regulation involves various mechanisms, including control at the levels of transcription, mRNA stability, and enzyme synthesis.

Therefore, studying how E. coli modulates these pathways provides insights into bacterial adaptability and gene regulation.

Gene Expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, usually proteins. This involves transcription of the gene into mRNA and then translation into proteins.
In E. coli, gene expression is tightly regulated to respond effectively to environmental cues, like the presence of nutrients or other chemical signals such as tryptophan.
The amount of tryptophan synthase activity in the cell is a direct result of gene expression regulation, which involves balancing both mRNA and protein stability.

• The rapid degradation of mRNA can lead to decreased enzyme levels, altering gene expression outcomes.
• On the other hand, stable proteins can maintain their activity, supporting ongoing metabolic processes even if new protein synthesis reduces.

Thus, efficient gene expression depends on the intricate balance of transcriptional control, mRNA, and protein stability.