Question

Illustrate and describe how the lack operon of Escherichia coli functions (include in your response what happens with a. high glucose/low lactose, b. high glucose/high lactose, c. low glucose/low lactose, and d. low glucose/high lactose).

Answer 1

The Lac Operon in E. coli operates by sensing both Glucose and Lactose levels. In order for the Lac Operon to be turned on, Glucose needs to be absent and Lactose needs to be present (although it is Allolactose, an isomer of Lactose that is required to allow the genes of the Lac operon to be transcribed).

The Lac Operon is comprised of three 'structural' genes. Thsese are:

• Lac Z: This codes for -Galactosidase, an enzyme that cleaves Lactose into Glucose and Galactose
• Lac Y: This codes for -Galactoside permease, a protein that allows Lactose to be imported into the cell.
• Lac A: This codes for -Galactoside transacetylase, an enzyme that transfers an Acetyl group from Acetyl CoenzymeA to Lactose and other sugars containing Galactose.

The other components of the Lac Operon include:

• A CAP binding site, a region upstream of the structural genes that CAP (Catabolite Activator Protein) binds to
• A promoter region, that binds RNA Polymerase prior to transcription
• The Operator region (the operator overlaps with the promoter) which is bound by Lac Repressor Protein when Lactose is absent.

The Lac Operon is turned on when glucose is not present and Lactose is available. When Lactose is present in the cell, some of the Lactose is converted into Allolactose, an isomer of Lactose. Allolactose is able to bind to the Lactose Repressor protein, which normally blocks the transcription of the Lac Operon. However, when Lactose levels are low, no Allolactose is present in the cell and Lac Repressor is able to bind to the Operator region of the Lac Operon and prevent Transcription.

The Lac Operon is also controlled by Glucose Levels in the cell. When Glucose levels are high, CAP is unable to bind to the CAP binding site. The CAP protein is an important Transcription factor as it recruits RNA Polymerase to the Lac promoter. However CAP binding to the CAP binding site is dependent on the presence of cyclic-Adenosine monophosphate (cAMP). Under high glucose conditions, cAMP levels are low, which in-turn prevents CAP binding to DNA. However, when Glucose levels are low, cAMP concentrations increase and cAMP-CAP is able to bind to the CAP binding site and recruit RNA Polymerase.