The trpL region contains four repeated DNA sequences that lead to the formation of stem-loop structures in mRNA. What are these stem-loop structures, and how do they affect transcription of the structural genes of the trp operon?

Verified step by step guidance

1. Understand the trp operon: The trp operon in bacteria is a group of genes involved in the biosynthesis of tryptophan. It is regulated by transcriptional attenuation, which involves the trpL region.
2. Identify the trpL region: The trpL region is a leader sequence located upstream of the structural genes. It contains four repeated DNA sequences (regions 1, 2, 3, and 4) that can form stem-loop structures in the mRNA transcript.
3. Explain the stem-loop structures: The stem-loop structures are formed by complementary base pairing between these regions. Specifically, regions 1 and 2 can pair, regions 2 and 3 can pair, and regions 3 and 4 can pair. The pairing of regions 3 and 4 forms a terminator stem-loop, which halts transcription.
4. Describe the role of tryptophan levels: When tryptophan levels are high, the ribosome quickly translates the leader peptide encoded by the trpL region, allowing regions 3 and 4 to pair and form the terminator stem-loop. This stops transcription of the structural genes.
5. Explain the alternative pairing: When tryptophan levels are low, the ribosome stalls at the trp codons in region 1, allowing regions 2 and 3 to pair instead. This prevents the formation of the terminator stem-loop, allowing transcription of the structural genes to proceed.

Key Concepts

Stem-Loop Structures

Stem-loop structures are secondary RNA structures formed when a single strand of RNA folds back on itself, creating a double-stranded 'stem' and a loop. These structures play a crucial role in the regulation of gene expression, particularly in prokaryotes, by influencing the stability of mRNA and its interaction with ribosomes.

trp Operon

The trp operon is a group of genes in bacteria that are involved in the biosynthesis of the amino acid tryptophan. It is regulated by a feedback mechanism where the presence of tryptophan inhibits the transcription of the operon, allowing the cell to conserve resources when tryptophan is abundant.

Transcription Termination

Transcription termination is the process by which RNA polymerase stops synthesizing RNA and detaches from the DNA template. In the trp operon, the formation of stem-loop structures in the mRNA can signal RNA polymerase to terminate transcription prematurely, thereby preventing the synthesis of downstream structural genes when tryptophan levels are sufficient.