​A lac-lambda hybrid system is developed to study the λ-repressor protein, in which the λ Cl gene is under the control of E. coli lac promoter and operator, and the lacZ gene is under the control of λ-PRM promoter and OR operator of λ-phage. Both the plasmids are introduced in E. coli and the concentrations of the proteins are determined upon the addition of IPTG.​

​Which graph correctly represents the expected results?​

Correct option is C

Explanation:

• IPTG induces the lac promoter controlling the λ CI gene, so λ repressor protein levels increase with increasing IPTG concentration (dashed line).

• As the λ repressor increases, it activates the λ-PRM promoter, increasing lacZ expression as well (solid line).

• This results in a sigmoidal increase in LacZ protein concurrent with increased λ repressor levels, depicted accurately by graph C.

• Graph D showing LacZ decreasing is incorrect since λ repressor here acts as an activator of the PRM promoter.

Information Booster:

• The lac promoter/operator is inducible by IPTG, which activates expression of the λ CI repressor in this system.

• The λ repressor can activate its own promoter (PRM), leading to increased lacZ expression in this hybrid construct.

• This system is used to study positive and negative regulation in gene expression.