Question

Microbiology question

51. Lacl regulates the expression of the lac operon, which encodes enzymes necessary for lactose metabolism. The genotypes of different E. coli strains are listed below. (For this exercise, lacP refers to the promoter, and lacO refers to the operator site where Lacl binds. lacl+ means wild-type, whereas lacl- indicates a loss-of-function mutation. lacO- means that Lacl cannot bind the operator.) For each strain, indicate whether the LacZ protein will be synthesized under the conditions indicated, when glucose level is low. Use + to indicate that LacZ activity will be detected and - to indicate otherwise. (3 Genotype lacl+ lacP+ laco+ lacZ+ lacl- lacP lacO+ lacZ+ lacl+ lacP+ lacO- lacZ+ Lactose present Lactose absent

Answer 1

Lac-operon is a repressible operon in nature.This operon consists of an inducer,a promoter,an operator and the cist ron genes.There also occurs a repressor molecules which becomes inactive in the present of substrate i.e.. lactose in this case.This means that when lactose is present,the repressor becomes inactive and cannot bind to the operator and mask it for linear movement of RNA polymerase, thus transcription and translation proceeds.Absence or silencing of either of these DNA sequences results in altogether halting of transcription as RNA polymerase fails to bind to the operon.

Thus, answer can be tabulated as below:

Genotype	Lactose Present	Lactose absent
case 1	Activity detected(repressor inactive,transcription takes place)	no activity (repressor active,binds perator and no transcription takes place)
case 2	No activity (Inducer is absent,so no binding of RNA polymerase to operon takes place)	no activity(Inducer is absent ,so no binding of RNA polymerase to operon takes place)
case 3	No activity(Operator or the master switch of operon is absent,so no binding of RNA polymerase to operon takes place)	no activity (Operator or the master switch of operon is absent,so no binding of RNA polymerase to operon takes place)