Gene Control: Lac Operon (Cambridge (CIE) A Level Biology): Revision Note

Gene control: lac operon

• Regulatory genes control structural genes and their levels of protein production

• Regulatory genes sometimes have control over several structural genes at once

• Structural genes in prokaryotes can form an operon: a group or a cluster of genes that are controlled by the same promoter

• The lac operon found in some bacteria is one of the most well-known of these

• The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins

• Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell

• It is an inducible enzyme that is only synthesised when lactose is present

• This helps prevent the bacteria from wasting energy and materials

Structure of the lac operon

• The components of the lac operon are found in the following order:

  • Promoter for structural genes

  • Operator

  • Structural gene lacZ that codes for lactase

  • Structural gene lacY that codes for permease (allows lactose into the cell)

  • Structural gene lacA that codes for transacetylase

• Located to the left (upstream) of the lac operon on the bacterium's DNA there is also the:

  • Promoter for regulatory gene

  • Regulatory gene lacI that codes for the lac repressor protein

• The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)

  • When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter

  • When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator

When lactose is absent

• The following processes take place when lactose is absent in the medium that the bacterium is growing in:

  • The regulatory gene is transcribed and translated to produce lac repressor protein

  • The lac repressor protein binds to the operator region upstream of lacZ

  • Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region

  • Transcription of the structural genes does not take place

  • No lactase enzyme is synthesized

When lactose is present

• The following processes take place when lactose is present in the medium that the bacterium is growing in:

  • There is an uptake of lactose by the bacterium

  • The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site

  • RNA polymerase is then able to bind to the promoter region and transcription takes place

  • The mRNA from all three structural genes is translated

  • Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium