Question

As described in Chapter 13, enzymes known as aminoacyl-tRNA synthetases are responsible for attaching amino acids to tRNAs. Let's suppose that in a mutant bacterium tryptophanyl-tRNA synthetase has a reduced ability to attach tryptophan to tRNA: its activity is only \(10 \%\) of that found in a normal bacterium. How would attenuation of the \(\operatorname{trp}\) operon be affected? Would the operon be more or less likely to be attenuated? Explain your answer.

Short answer

Due to the reduction in the enzyme's ability, the attenuation of the trp operon would be more likely because tryptophan production would decrease, leading to fewer trp-tRNA molecules available for translating the trp operon.

Step-by-step solution

Understand the problem

Firstly, understand what tryptophanyl-tRNA synthetase does. It attaches amino acid tryptophan to tRNA. However, in the given situation, its capability is reduced to only 10% that of a normal bacterium.

Analyze the effects of lower enzyme activity

Analyze what would happen when the activity of tryptophanyl-tRNA synthetase enzyme is reduced. This likely means there would be fewer trp-tRNA molecules available. Consequently, ribosomes would be slower in translating the trp operon, leading to a decreased rate in protein synthesis.

Understand the impact on attenuation of the trp operon

The trp operon itself is a set of genes that produce the enzymes needed to make tryptophan. Attenuation is the premature termination of transcription. With the reduced enzyme ability, there would be a lower concentration of tryptophan, and thus decreased trp-tRNA levels. Therefore, further attenuation should be expected, making it less accessible for tryptophan production.

Final assessment

Thus, with reduced enzyme activity, the trp operon would be more likely to be attenuated due to a lower level of tryptophan present. This reduced ability impacts the cell's capacity to produce proteins and hence, would cause an increase in the attenuation of the trp operon.

Key concepts

Aminoacyl-tRNA Synthetases

Aminoacyl-tRNA synthetases are crucial enzymes in the process of translation, which is part of protein synthesis. Essentially, they ensure that the correct amino acids are attached to their corresponding transfer RNAs (tRNAs). This is imperative for accurate translation of the genetic code from mRNA into a chain of amino acids, leading to protein formation.

Each amino acid has its specific synthetase. For instance, tryptophanyl-tRNA synthetase attaches the amino acid tryptophan to its tRNA. If this enzyme's activity is compromised as in the provided exercise scenario, it would struggle to maintain adequate levels of charged tRNA. With only 10% functional efficiency, fewer tRNAs would be available to insert tryptophan into the growing polypeptide chains. This would disrupt normal protein synthesis, potentially resulting in a broad range of cellular dysfunctions.

Genetic Mutation Effects

Genetic mutations can significantly affect a cell's ability to perform essential functions. In the case of the exercise, a mutation that reduces the efficiency of tryptophanyl-tRNA synthetase can have a cascade of effects. Since this enzyme is less capable of charging tRNAs with tryptophan, the mutation can lead to a deficit in the levels of tryptophan available for protein synthesis.

Such a mutation doesn't just affect the translation process directly; it can also impact transcriptional regulation mechanisms like attenuation. Specifically, in the trp operon, the availability of charged trp-tRNAs is a signal that dictates whether transcription should continue or be terminated early. The effect of this mutation, therefore, extends beyond the enzymatic activity and into the genetic regulation of tryptophan biosynthesis.

Tryptophan Biosynthesis

The biosynthesis of tryptophan is a biochemical pathway involving multiple enzymes encoded by the trp operon in bacteria like E. coli. These enzymes catalyze a series of reactions that convert precursor molecules into tryptophan. If a key component in this pathway, such as tryptophanyl-tRNA synthetase, is impaired, the whole tryptophan biosynthesis can be affected.

Normally, when tryptophan levels are adequate, the cell attenuates the trp operon to prevent the wasteful production of excess enzymes. However, if the synthesis of tryptophan is compromised due to a malfunctioning enzyme, the feedback inhibition mechanism might be triggered less, potentially leading to attenuation reduction. This is counter-intuitive as the cell actually needs more tryptophan but the regulatory systems can misinterpret the signals due to the mutation.

Transcriptional Regulation

Transcriptional regulation involves controlling the amount and timing of gene expression. In the trp operon, this is partly achieved through attenuation, a process where transcription is terminated early when tryptophan levels are high. The charged trp-tRNA plays a role in this mechanism by stalling or allowing the RNA polymerase to continue transcription based on the concentration of tryptophan.

In the scenario presented in the exercise, the impaired enzyme activity would lead to low levels of charged trp-tRNA, which means the operon would be less likely to undergo attenuation, thereby increasing RNA polymerase's transcription of the trp operon genes. This paradoxical increase in gene transcription during low tryptophan levels reflects the complexity of transcriptional regulation and highlights the delicate balance within cellular metabolism that can be disrupted by mutations.