Prokaryotic Gene Regulation

The trp Operon: A Repressible Operon

Bacteria such as Escherichia coli need amino acids to survive, and are able to synthesize many of them. Tryptophan is one such amino acid that E. coli can either ingest from the environment or synthesize using enzymes that are encoded by five genes. These five genes are next to each other in what is called the tryptophan (trp) operon (Figure). The genes are transcribed into a single mRNA, which is then translated to produce all five enzymes. If tryptophan is present in the environment, then E. coli does not need to synthesize it and the trp operon is switched off. However, when tryptophan availability is low, the switch controlling the operon is turned on, the mRNA is transcribed, the enzyme proteins are translated, and tryptophan is synthesized.

The trp operon has a promoter, an operator, and five genes named trpE, trpD, trpC, trpB, and trpA that are located in sequential order on the DNA. RNA polymerase binds to the promoter. When tryptophan is present, the trp repressor binds the operator and prevents the RNA polymerase from moving past the operator; therefore, RNA synthesis is blocked. In the absence of tryptophan, the repressor dissociates from the operator. RNA polymerase can now slide past the operator, and transcription begins.
The tryptophan operon. The five genes that are needed to synthesize tryptophan in E. coli are located next to each other in the trp operon. When tryptophan is plentiful, two tryptophan molecules bind the repressor protein at the operator sequence. This physically blocks the RNA polymerase from transcribing the tryptophan genes. When tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed.

The trp operon includes three important regions: the coding region, the trp operator and the trp promoter. The coding region includes the genes for the five tryptophan biosynthesis enzymes. Just before the coding region is the transcriptional start site. The promoter sequence, to which RNA polymerase binds to initiate transcription, is before or “upstream” of the transcriptional start site. Between the promoter and the transcriptional start site is the operator region.

The trp operator contains the DNA code to which the trp repressor protein can bind. However, the repressor alone cannot bind to the operator. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes the shape of the repressor protein to a form that can bind to the trp operator. Binding of the tryptophan–repressor complex at the operator physically prevents the RNA polymerase from binding to the promoter and transcribing the downstream genes.

When tryptophan is not present in the cell, the repressor by itself does not bind to the operator, the polymerase can transcribe the enzyme genes, and tryptophan is synthesized. Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is said to be negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators.

Link to Learning

Watch this video to learn more about the trp operon.

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