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The trp operon is a group of genes that are transcribed together, encoding the enzymes that produce the amino acid tryptophan in bacteria. The trp operon was first characterized in Escherichia coli , and it has since been discovered in many other bacteria. [ 1 ]
The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator. The trp operon is active only when cellular tryptophan is scarce. If there isn't enough tryptophan, the repressor protein breaks off from the operator (where the repressor is normally bound) and RNA polymerase can complete its reading of the strand of DNA ...
For example, the E. coli tryptophan repressor (TrpR) is only able to bind to DNA and repress transcription of the trp operon when its corepressor tryptophan is bound to it. TrpR in the absence of tryptophan is known as an aporepressor and is inactive in repressing gene transcription. [2]
The trp operon, involved in the synthesis of tryptophan (which itself acts as the corepressor), is a negatively controlled repressible operon. Operons can also be positively controlled. With positive control, an activator protein stimulates transcription by binding to DNA (usually at a site other than the operator).
The GAL4/UAS system is an example of both an inducible and repressible system. Gal4 binds an upstream activation sequence (UAS) to activate the transcription of the GAL1/GAL7/GAL10 cassette. On the other hand, a MIG1 response to the presence of glucose can inhibit GAL4 and therefore stop the expression of the GAL1/GAL7/GAL10 cassette. [42]
RNA polymerase can then begin to transcribe operon genes. By binding to activators. Activators generally bind poorly to activator DNA sequences unless an inducer is present. Activator binds to an inducer and the complex binds to the activation sequence and activates target gene. [2] Removing the inducer stops transcription. [2]
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The inducible enzyme is used for the breaking-down of things in the cell. It is also a part of the Operon Model, which illustrates a way for genes to turn "on" and "off". The inducer causes the gene to turn on (controlled by the amount of reactant which turns the gene on). Then there's the repressor protein that turns genes off.