16.3.3—Genetic control in prokaryotes

In a prokaryotic operon, a promoter, operator and linked structural genes are transcribed together. A regulatory protein can bind the operator and prevent RNA polymerase from transcribing the structural genes.

Promoter: RNA-polymerase binding site; operator: regulatory switch; structural genes: pathway products; regulator gene: produces the repressor or activator; a small molecule can alter regulator activity.

Co-transcription ensures enzymes in the same pathway are produced together and reduces wasted resources. The operator connects a signal to a whole group of genes.

When a required substrate is absent, a repressor can block the operator; when the substrate appears, it changes the repressor so transcription proceeds.

The operator is DNA, not an enzyme, and the structural genes do not each need a separate promoter in a simple operon model.