Question

Can someone please explain the following lac operon mutation regulatory effect on transcription:

1) lacO^CZ⁺Y⁺/lacO⁺Z⁺Y⁺ = Lac^c

2) lacO^CZ^-Y^-/lacO⁺Z⁺Y⁺ = Lac⁺

3) lacI^-D/lacI⁺ = Lac^c

4) lacI^S/lacI⁺ = Lac^-

Answer 1

1)

there is mutation in operator site (lacOc) which does not allow the binding of lac repressor so lac operon will never be turned off which results in constitutive expression of lac operon.

2)

Even though there is constitutive mutation (lacOc) on operator site of one operon in partially diploid cell, no constitutive expression will be observed as there is no wild type lacZ and lacY present. So expression of lac operon in this cell will be observed based on other lac operon in the cell which is wild type lac operon

3)

lacL-D mutation is the mutation in lacI protein which cannot bind to the operator site of lac operon. Which results in constitutive expression of lac operon

4)

laclS mutation is the mutation in lacl protein which cannot bind to the allo-lactose, so once lacl protein binds to the operator site it cannot be removed from there as inducer molecule (allo-lactose) cannot bind to it in order to disassociate lacI protein. This mutant protein will bind to both the operator site which turn off the lac operons.