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4. 12: Prokaryotic Gene Regulation

On or off?

When it comes to genes, that is an important question. And if you're a single-celled organism like a bacterium, conserving energy by not producing unnecessary proteins is very important.

Prokaryotic Gene Regulation

Transcription is regulated differently in prokaryotes and eukaryotes. In general, prokaryotic regulation is simpler than eukaryotic regulation.

The Role of Operons

Regulation of transcription in prokaryotes typically involves operons. An operon is a region ofDNA that consists of one or more genes that encode the proteins needed for a specific function. The operon also includes a promoter and an operator. The operator is a region of the operon where regulatory proteins bind. It is located near the promoter and helps regulate transcription of the operon genes.

The Lac Operon

A well-known example of operon regulation involves the lac operon in E. coli bacteria (see Figure below and the video at the link below). The lac operon consists of a promoter, an operator, and three genes that encode the enzymes needed to digest lactose, the sugar found in milk. The lac operon is regulated by lactose in the environment. The Lac Operon video at explains the operon in further detail.

  • When lactose is absent, a repressor protein binds to the operator. The operator is located between the promoter and the three lac operon genes. The protein blocks the binding of RNA polymerase to the promoter. As a result, the lac genes are not expressed.
  • When lactose is present, the repressor protein does not bind to the operator. This allows RNA polymerase to bind to the promoter and begin transcription. As a result, the lac genes are expressed, and lactose is digested.

Why might it be beneficial to express genes only when they are needed? (Hint: synthesizing proteins requires energy and materials.)

The parts of the lac operon: promoter, operator, lacZ, lacY, lacA, terminator

The three genes of the lac operon are lacZ, lacY, and lacA. They encode proteins needed to digest lactose. The genes are expressed only in the presence of lactose.


  • Regulation of transcription in prokaryotes typically involves an operon, such as the lac operon in E. coli.
  • The lac operon is regulated by proteins that behave differently depending on whether lactose is present or absent.

Explore More

Explore More I

Use this resource to answer the questions that follow.

  1. How do bacteria break large sugars into smaller ones?
  2. What is the role of lactose in gene regulation?
  3. What happens when lactose is present? Or absent?
  4. What is the operator?
  5. What does "operon" refer to?

Explore More II

Gene Machine: The Lac Operon at


  1. What is an operon?
  2. Why might it be beneficial to express genes only when they are needed?
  3. What is the role of an operon's operator?
  4. What happens to the lac operon in the absence of lactose?
  5. Draw a diagram to show how the lac operon is regulated.