Environmental Signals are Converted to Gene Action

Metabolism of the Lactose

Induction is the production of an enzyme or product due to the presence of some compound in the media. For example the enzyme beta-Galactosidase is produced by E. coli in the presence of lactose but only in the absence of glucose.

Why is Isopropyl-beta-thiogalactoside - IPTG is a much better inducer of beta-galactosidase?

Three gene (z, y, a) are coordinately regulated producing
three enzymes (beta-galactosidase, permease, and
trans-acetylase).

The repressor binds to the operator of the lac gene cluster in the absence of the inducer. If inducer is present the repressor is allosterically changed thus, preventing binding to the operator and the RNA polymerase is allowed to copy the DNA.

The glucose effect is related to cAMP (cyclic Adenosine Mono Phosphate) levels. High glucose yields low cAMP and conversely Low glucose yields high cAMP. The cAMP interacts with a protein called catabolite activator protein (CAP)

High glucose (--> low cAMP) + no lactose(IPTG) ---> no enzymes. 

High glucose (--> low cAMP) + lactose(IPTG) ---> no enzymes. 

Low glucose (--> High cAMP) + lactose(IPTG) ---> high enzymes.

Summary of Negative and Positive Control.

Invitro binding

Predicted cap binding