The function of tRNA synthetases is to attach amino acids to tRNAs. Suppose the tRNA synthetase responsible for attaching tryptophan to tRNA is mutated in a bacterial strain, with the result that the tRNA synthetase functions at about 15% of the efficiency of the wild-type tRNA synthetase. Would formation of the 3–4 stem-loop structure in mRNA be more frequent or less frequent in the mutant strain than in the wild-type strain? Why?

Verified step by step guidance

1. Understand the role of tRNA synthetases: tRNA synthetases are enzymes that attach specific amino acids to their corresponding tRNAs. This process is critical for accurate translation of mRNA into proteins. In this case, the tRNA synthetase in question is responsible for attaching tryptophan to its corresponding tRNA (tRNA^Trp).
2. Review the 3–4 stem-loop structure in mRNA: The 3–4 stem-loop structure is part of the attenuation mechanism in bacterial operons, such as the trp operon. When this structure forms, it causes transcription termination, preventing the downstream genes from being transcribed. The formation of this structure depends on the availability of charged tRNA^Trp.
3. Analyze the impact of the mutation: In the mutant strain, the tRNA synthetase functions at only 15% of the efficiency of the wild-type enzyme. This means that fewer tRNA^Trp molecules will be charged with tryptophan, leading to a lower availability of charged tRNA^Trp in the cell.
4. Relate the availability of charged tRNA^Trp to the 3–4 stem-loop formation: When charged tRNA^Trp is scarce, ribosomes translating the leader peptide sequence in the trp operon stall at the tryptophan codons. This stalling prevents the formation of the 3–4 stem-loop structure, allowing transcription of the downstream genes to continue. Conversely, if charged tRNA^Trp is abundant, the ribosome does not stall, and the 3–4 stem-loop forms, terminating transcription.
5. Conclude the effect of the mutation: In the mutant strain, due to the reduced efficiency of the tRNA synthetase, the formation of the 3–4 stem-loop structure would be less frequent compared to the wild-type strain. This is because the scarcity of charged tRNA^Trp increases ribosome stalling, preventing the formation of the termination structure.

Key Concepts

tRNA Synthetases

tRNA synthetases are enzymes that play a crucial role in protein synthesis by attaching specific amino acids to their corresponding tRNA molecules. Each tRNA synthetase is specific to one amino acid and its corresponding tRNA, ensuring that proteins are synthesized accurately according to the genetic code. A mutation that reduces the efficiency of a tRNA synthetase can lead to decreased availability of the amino acid for protein synthesis, potentially affecting the overall translation process.

Stem-Loop Structures in mRNA

Stem-loop structures in mRNA are formed when complementary sequences within the RNA strand base-pair with each other, creating a loop. These structures can play significant roles in the regulation of gene expression, including the termination of transcription and the modulation of translation. The frequency of stem-loop formation can be influenced by the availability of tRNAs and the efficiency of translation, as they can affect the stability and processing of the mRNA.

Translation Efficiency

Translation efficiency refers to how effectively ribosomes synthesize proteins from mRNA, which is influenced by the availability of tRNAs and their corresponding amino acids. If a tRNA synthetase is functioning at reduced efficiency, it may lead to a shortage of the associated amino acid, thereby slowing down the translation process. This reduction can impact the formation of secondary structures in mRNA, such as stem-loops, as the ribosome may pause more frequently during translation, allowing more time for these structures to form.