Question

3. Mutations in bacteria that affect gene regulation may have a cis- or trans-effect on the expression of the protein-encoding genes in an operon? How would the following mutations affect the expression of the lac operon? Assume there could be more than one lac operon in the cell and that no glucose is present.

  A. A mutation in the operator site that prevents the lac repressor from binding.

  B. A mutation in the lacI gene that prevents allolactose from binding, but the repressor can still bind to the operator sites just fine.

  C. A mutation in the lacI gene that prevents the lac repressor from binding to the operator sites.

Answer 1

A. If the repressor will not bind to the operator region of Lac Operon then the Lac Operon will be constitutively expressed irrespective of the presence or absence of lactose in the medium.

B. If the repressor cannot bind to allolactose, then degradation of repressor will not take place and it will be permanently bound to the operator region. In this case, the Lac Operon will always be in a switched off condition irrespective of the presence or absence of lactose in the medium.

C. If the repressor cannot bind to the operator region, then the Lac Operon will never be in a switched off condition and will therefore always be in a switched on condition. It will be constitutively expressed.

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