Question

What is the mechanism by which glucose affects the induction of ß-galactosidase by lactose?

Answer 1

The metabolism of lactose is under the control of the Lac-operon. An operon is a functioning unit of genomic DNA controlled by a single promoter.

The lac- operon contains genes that code for proteins which transport lactose into the cytoplasm and digesting it into glucose by production of beta- galactosidase. Normally, the lac operon is turned off. The regulatory gene produces the repressor molecule, which binds to the opertor site, preventing transcription.

When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon. The inducer binds to the repressor forming a repressor-inducer complex which is inactive and now, the repressor do not bind to the operator region. So the RNA polymerase can now transcribe the structural genes. The Lac operon is then transcribed and translated into proteins, facilitating lactose transport into the cell, and its digestion into glucose by the beta-galactosidase enzyme.

How glucose affects this ???

The glucose level in the cell should be low for the above process to occur. When glucose levels are low, the level of  cyclic adenosine monophosphate (cAMP) levels increase. Whenrhe levels of cAMP are sufficient, the cAMP bind and activate another protein, called catabolite activator protein (CAP). CAP now binds to the operon which helps RNA polymerase to attach to the promoter region. This leads to RNA polymerase transcription of genes leading to an increase of lac operon expression.

So, when glucose in the cell is high, there is no CAP protein. So, RNA polymerase cannot bind to the promoter region. So there is no transcription of the Lac operon.