Figure 16.13 depicts numerous critical regions of the leader sequence of mRNA that play important roles during the process of attenuation in the trp operon. A closer view of the leader sequence, which begins at about position 30 downstream from the 5' end, is shown below, running along both columns. Within this molecule are the sequences that cause the formation of the alternative hairpins. It also contains the successive triplets that encode tryptophan, where stalling during translation occurs. Take a large piece of paper (such as manila wrapping paper) and, along with several other students from your genetics class, work through the base sequence to identify the trp codons and the parts of the molecule representing the base-pairing regions that form the terminator and antiterminator hairpins shown in Figure 16.13.

Key Concepts

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 through a mechanism called attenuation, which allows the cell to respond to tryptophan levels by controlling the transcription of the operon. When tryptophan is abundant, the operon is turned off, while low levels of tryptophan lead to its activation, demonstrating a classic example of gene regulation.

mRNA Leader Sequence

The leader sequence of mRNA is a short region at the beginning of the mRNA transcript that plays a crucial role in the regulation of gene expression. In the context of the trp operon, this sequence contains specific codons for tryptophan and is responsible for forming secondary structures, such as hairpins, that influence whether transcription continues or terminates. The formation of these structures is sensitive to the availability of tryptophan during translation.

Hairpin Structures

Hairpin structures are formed when complementary sequences within a single strand of RNA base-pair with each other, creating a loop. In the trp operon, the formation of alternative hairpins in the leader sequence determines whether transcription will terminate or proceed. The presence of a terminator hairpin signals the RNA polymerase to stop transcription, while an antiterminator hairpin allows transcription to continue, thus playing a critical role in the regulation of gene expression.