Question

How does the lac repressor affect transcription of the lac operon

Answer 1

• LAC operon of E. coli is an example of inducible gene expression system.
• Lac operon includes:

1. Controlling genes:

• Promoter- Region for binding of RNA polymerase to initiate transcription of the structural genes present down-stream of the promoter.
• Operator- Region where the repressor binds

2. Structural genes-

• Lac Z- encodes Beta galactosidase.
• Lac Y- encodes permease,
• Lac A- encodes transacetylase.

• Lac i represents the repressor and is under a separate promoter and not under lac promoter control.
• Lac repressor can recognize three regions of operator.

O1 is present within the promoter.

O2 is present within lacZ gene 401 bp downstream of O1.

O3 in present in lac I gene, which is 93 bp upstream of O1.

• Lac repressor is a tetrameric structure.
• With two of its subunits it binds to O1 and the other two forms loop with O2 or O3.
• In absence of inducer, lac repressor remains bound to the operators. This prevents the RNA polymerase to transcribe the structural genes.
• Lactose (Allolactose) acts as natural inducer.
• Lac repressor has binding site for the inducer.
• Thus, in presence of the inducer, Lac repressor binds with the inducer. This results in a conformational change in the repressor and the repressor does not bind to the operators.
• Thus, the transcription os the structural genes occurs.