During development, gene expression must be carefully regulated to ensure that the right genes are expressed only at the correct time and in the correct place. Gene expression in eukaryotes and bacteria is often regulated at the transcription stage. Control of other levels of gene expression is also important. RNA molecules play many roles in regulating eukaryotic gene expressions.
When the repressor binds to the operator in blocks the transcription of RNA The trp repressor is the product of a regulatory gene called trpR , which is located at some distance from the operon it controls and has its own promoter. Regulatory genes are transcribed continuously at slow rates, and a few trp repressor molecules are always present in an E. coli cell.
By itself, an operon is turned on. RNA polymerase can bind to the promoter and transcribe the genes
When an E. coli cell must make tryptophan for itself, all the enzymes are synthesized at one time. The switch is a segment of DNA called an operator. The operator, located between the promoter and the enzyme-coding genes, controls the access of RNA polymerase to the genes.
Purple is the DNA which transcribes to make the wiggly line mRNA which translates to make the repressor protein in red which is active. It then binds to the operator so that RNA polymerase can’t bind to the operator and no DNA past the promoter will be transcribed. If the repressor is bonud it becomes inactive, not binding to the operator, and allowing transcription to take place to produce enzymes to break down the substance that bound to the repressor in the first place.
Natural selection favors bacteria that express only those genes whose products are needed by the cell. A bacterium in a tryptophan-rich environment that stops producing tryptophan conserves its resources.
When lactose is absent, the repressor is active, and the operon is off (operon = promoter, operator, and structural genes). The lac repressor is active on its own, and in the absence of lactose it switches off the operon by binding to the operator.
When lactose is present, allolactose (an isomer of lactose) binds to the repressor making it inactive which allows the operon to be on so the enzymes for lactose utilization are induced.
The ancient bacteria that could do this had an evolutionary advantage over those that couldn’t.
The trp operon is an example of a repressible operon, one that is inhibited when a specific small molecule (tryptophan) binds allosterically to a regulatory protein.
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