The regulation of gene expression in prokaryotic cells occurs at the transcriptional level. There are three ways to control the transcription of an operon: repressive control, activator control, and inducible control. Repressive control, typified by the trp operon, uses proteins bound to the operator sequence to physically prevent the binding of RNA polymerase and the activation of transcription. Therefore, if tryptophan is not needed, the repressor is bound to the operator and transcription remains off. Activator control, typified by the action of CAP, increases the binding ability of RNA polymerase to the promoter when CAP is bound.
In this case, low levels of glucose result in the binding of cAMP to CAP. CAP then binds the promoter, which allows RNA polymerase to bind to the promoter better. In the last example—the lac operon—two conditions must be met to initiate transcription. Glucose must not be present, and lactose must be available for the lac operon to be transcribed. If glucose is absent, CAP binds to the operator. If lactose is present, the repressor protein does not bind to its operator. Only when both conditions are met will RNA polymerase bind to the promoter to induce transcription.
protein that binds to prokaryotic operators to increase transcription
catabolite activator protein (CAP)
protein that complexes with cAMP to bind to the promoter sequences of operons that control sugar processing when glucose is not available
operon that can be activated or repressed depending on cellular needs and the surrounding environment
operon in prokaryotic cells that encodes genes required for processing and intake of lactose
protein that prevents transcription
region of DNA outside of the promoter region that binds activators or repressors that control gene expression in prokaryotic cells
collection of genes involved in a pathway that are transcribed together as a single mRNA in prokaryotic cells
protein that increases transcription
protein that binds to the operator of prokaryotic genes to prevent transcription
transcriptional start site
site at which transcription begins
series of genes necessary to synthesize tryptophan in prokaryotic cells
amino acid that can be synthesized by prokaryotic cells when necessary