Trp Attenuation: A Clever Way to Regulate
Prokaryotic cells do not have as many forms of transcriptional/ translational genetic regulation that eukaryotic cells have. The nuclear membrane in eukaryotes allows those cells to avoid concurrent transcription and translation. But there still is a very clever solution to genetic regulation in prokaryotes.
Along with a repressor protein, that when bound to tryptophan enhances its ability to bind to the operator region of the trp operon (blocking the RNAP from transcribing the operon), the bacterial cell also utilizes attenuation.
The trp operon yields gene products that play an enzymatic role in the biosynthesis of tryptophan. This operon contains 7 genes: 1 containing the promoter & operator region (the operator is contained within the promoter), 1 leader sequence and 5 genes that play the active role in biosynthesis of tryptophan. Prior to the 5 genes that encode the enzymes that synthesize tryptophan there is the leader region that also gets transcribed. This leader region encodes 14 amino acids which includes two consecutive tryptophan residues. The leader region along with the 5 genes are transcribed into a polycistronic mRNA. Unique only to prokaryotes this is an mRNA that will yield more than one protein.
The mRNA is divided into 4 domains. Domain 3 of the mRNA is able to base pair with either domain 2 or domain 4. Domain 1 includes the leader sequence along with the attenuator; the section that will require 2 consecutive trp amino acids during translation. If Domain 3 base pairs with domain 4 a stem loop structure will form resulting in the termination of the transcriptional/translational process.
When trp levels are high in the cell the ribosome translating the mRNA quickly covers domains 1 & 2, allowing domain 3 to base pair with domain 4 halting the process and ultimately preventing any more tryptophan from being synthesized. If trp levels are high the ribosome is able to quickly use a trp-charged tRNA to add to the growing peptide. When trp levels are low the ribosome stalls at domain 1 because trp-charged tRNAs are in short supply, which allows domain 2 to associate with domain 3. This prevents the termination step loop that forms when domain 3 associates with domain 4. In this situation the ribosome is able to continue translating the mRNA to yield the enzymes that will synthesize tryptophan.
A clever and intuitive solution to prevent a needless energy drain that would stress the cell.