Question

Many mutations have been identified in the lac operon. A particular mutation in the CRP gene make the CRP protein bind to the Plac promoter independent of the level of cAMP. Predict the expression of the lac operon in cells with this mutation under the different growth conditions by checking all conditions when the lac operon will be "on". (for the purposes of this question, either a extremely low level of expression or completely off are equivalent) Select one or more:
a. + glucose, + lactose
b. - glucose, - lactose
c. + glucose, - lactose
d. - glucose, + lactose

Answer 1

Answer:

a. + glucose, + lactose  = On

b. - glucose, - lactose  = Off

c. + glucose, - lactose  = Off

d. - glucose, + lactose = On

Explanation:

Lac operon has both types of control, repressible and inducible.

Whenever glucose level is low in the cell, an enzyme known as adenylyl cyclase raises the level of cAMP which forms a dimer with CRP protein and they both act as activator of lac operon and cause expression.

Apart from this, when lactose is present in the cell, β-galactosidase enzyme metabolizes lactose to form allolactose which causes allosteric repulsion in the lac repressor and causes its removal from the operator. As soon as repressor is removed lac operon gets activated.

In wild type lac operons, the expression of lac operon occurs when glucose level is low in the cell and lactose is present but in this mutant presence or absence of glucose will not make a difference because CRP will bind Plac promoter independent of cAMP level i.e. activator CRP will work even in high glucose concentration. If lactose is present then lac operon will always express so in option 'a & d' lac operon will express but in option 'b & c' it will not express.