7.18B: The trp Operon - A Repressor Operon
- Explain the relationship between structure and function of an operon and the ways in which repressors regulate gene expression
Bacteria such as E. coli need amino acids to survive. Tryptophan is one such amino acid that E. coli can ingest from the environment. E. coli can also synthesize tryptophan using enzymes that are encoded by five genes. These five genes are next to each other in what is called the tryptophan (trp) operon. If tryptophan is present in the environment, then E. coli does not need to synthesize it; the switch controlling the activation of the genes in the trp operon is turned off. However, when tryptophan availability is low, the switch controlling the operon is turned on, transcription is initiated, the genes are expressed, and tryptophan is synthesized.
A DNA sequence that codes for proteins is referred to as the coding region. The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon. Just before the coding region is the transcriptional start site. This is the region of DNA to which RNA polymerase binds to initiate transcription. The promoter sequence is upstream of the transcriptional start site. Each operon has a sequence within or near the promoter to which proteins (activators or repressors) can bind and regulate transcription.
A DNA sequence called the operator sequence is encoded between the promoter region and the first trp-coding gene. This operator contains the DNA code to which the repressor protein can bind. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes shape to bind to the trp operator. Binding of the tryptophan–repressor complex at the operator physically prevents the RNA polymerase from binding and transcribing the downstream genes.
When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized. Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators.
Key Points
- The operator sequence is encoded between the promoter region and the first trp-coding gene.
- The trp operon is repressed when tryptophan levels are high by binding the repressor protein to the operator sequence via a corepressor which blocks RNA polymerase from transcribing the trp-related genes.
- The trp operon is activated when tryptophan levels are low by dissociation of the repressor protein to the operator sequence which allows RNA polymerase to transcribe the trp genes in the operon.
Key Terms
- repressor : any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
- operon : a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together