Question

. β-Galactosidase (lacZ) has bifunctional activity. It hydrolyzes lactose to galactose and glucose and catalyzes the intramolecular isomerization of lactose to allolactose. β-Galactosidase promotes the isomerization by means of an acceptor site that binds glucose after its cleavage from lactose and thus delays its exit from the site. Allolactose, not lactose, is the natural inducer for the lac operon. a. Would you expect β-galactosidase to be induced in a zy+ mutant upon the addition of lactose? Upon the addition of allolactose? b. Would you expect β-galactoside permease to be induced in a z+ y- mutant upon the addition of lactose? Upon the addition of allolactose? c. Can you suggest a reason why cells synthesize low levels of β-galactosidase and permease even when there is absolutely no lactose in the medium?

Answer 1

There are namely 2 genes here one is the lac Z that synthesises the -galactosidase and another one is the lac Y that synthesises the permease protein. So in case of the ZY⁺ now the z should be either + or - if the z is + that means the the gene for the galatosidase is present and it signifies that the gene can be induced if the lactose is present in the medium and similarly upon the addition of allolactose the gene will also get induced becase if lactose is added some amount of the lactose gets converted to form the allolactose that works as the inducer by binding to the repressor.

Now if the type is z^- that means the gene for the galactosidase is absent and there is no chance of it getting induced even when lactose or allolactose is added.

IN this case the type is z⁺y^- here the y is negative that signifies that that the gene encoding the permease protein is absent or mutated in a loss of function way so no matter what is added to the ,medium, lactose or allolactose there will be no induction of the gene permease as it is mutated or absent.

The reason for the low level of synthesis or basal level of synthesis of galactosidase and permease is because in the Prokaryotik system the genes are placed under the control of a STRONG PROMOTER and when not needed the promoter is repressed by the repressor. So as the genes are present under the control of a strong promoter so there will always be low level or basal level transcription of those genes even when the repressor are present.