The Tryptophan Operon and the Lac Operon
An operon is a set of genes that are transcribed into a single mRNA by a single promoter. These operons are only found in prokaryotes since genes in eukaryotes are transcribed and regulated individually. The gene expression is controlled by the repressor and the activator. The repressor protein switches off the genes, while the activator proteins work with the promoter to turn on the genes. Operons are transcription switches that allow the cell to respond to changes in their environment.
With this information, one can understand the functions of the tryptophan operon. Inside the promoter is a short DNA sequence that is recognized by the transcription factor, tryptophan repressor. The binding of this protein, known as the operator, to the nucleotide sequence blocks the RNA polymerase from binding to the promoter. Thus, this prevents the transcription of the operon and the creation of the tryptophan-producing enzymes. The tryptophan repressor can only binds to the DNA if there are several molecules of the amino acid tryptophan bound to it. It is also an allosteric protein since the binding of tryptophan can cause changes to its structure in order to allow the protein to bind to the operator sequence. When the concentration of tryptophan within the cell drops, the repressor does not bind to tryptophan or the DNA and the gene is turned on. This allows the tryptophan operon to be transcribed and permit the production of the tryptophan-producing enzyme. When there is a rise in the tryptophan concentration, the always present tryptophan repressor binds to the tryptophan and the DNA and the gene is turned off. Therefore, it stops the production of tryptophan-producing enzymes.
Sometimes, the activity of the promoter can be controlled by two different transcription regulators, such as that of the Lac operon in E. coli. The Lac operon is regulated by both the Lac repressor and the CAP activator protein. The Lac operon encodes for proteins that are needed to import and digest the disaccharide lactose. In the absence of glucose, CAP turns on gens that allow the cells to use alternative sources of carbon like lactose. However, if CAP turns on the Lac operon without the present of lactose, it would be wasteful so the Lac repressor shuts off the Lac operon in the absence of lactose. This allows the operon to be expressed only when lactose is present and glucose is absent. When there is glucose and lactose present, the operon is turned off since CAP is not bound to it. With the present of glucose and the absence of lactose, the operon is turned off since the Lac repressor is bounded but CAP is not bound. When there is an absence of both glucose and lactose, the CAP is bound but so is the Lac repressor thus the operon is off. Therefore, with the absence of glucose and the presence of lactose, only CAP is bound to the sequence and the Lac operon is turned on.
In both the Lac operon and the tryptophan operon, the repressors play a major role in regulating gene expression. In the tryptophan operon, the regulation of the tryptophan concentration is controlled by the allosteric tryptophan repressor. In the Lac operon, the activities of the promoter are controlled by an activator and a repressor. The arrangement allows the operon to incorporate two different signals, thus, two different conditions must be met in order for the gene to be expressed. With this, one can understand how operons play a key part in gene expression.