Question

Imagine an E. coli cell that has a mutation that keeps catabolite activator protein (CAP) from forming properly. What would happen to this cell? A. Glucose would accumulate, galactose would accumulate B. Galactose would accumulate, lactose would accumulate C. Galactose would accumulate, lactose concentration would decrease D. Lactose would accumulate, glucose concentration would decrease

Answer 1

To have an answer in this problem we need to recap the lac operon. here is the diagram -

Jacob and Monod in 1961 described lac operon on E.coli The structural gene for β-galactosidase (lacZ) is clustered with the genes responsible for the permeation of lactose into the cell (lacY) and for thiogalactoside transacetylase (lacA). The structural genes for these three enzymes, along with the lac promoter and lac operator (a regulatory region), are physically associated to constitute the lac operon as depicted in above picture. This genetic arrangement of the structural genes and their regulatory genes allows for coordinate expression of the three enzymes concerned with lactose metabolism.

From the picture it is clear that in a situation in the environment where glucose is depleted cAMP-CAP protein -protein interaction with RNA Polymerase would increase the utilisation as lactose. When E coli is exposed to both lactose and glucose as sources of carbon, the organisms first metabolize the glucose and then temporarily stop growing until the genes of the lac operon become induced to provide the ability to metabolize lactose as
a usable energy source. This mechanism known as catabolite repression, is mediated by a catabolite gene activator protein (CAP) in conjunction with cAMP. In order for the RNA polymerase to recognise promoter site most efficiently, the cAMP-CAP complex must also be present in the cell.

So, without CAP lactose will never be utilised even when the glucose source is depleted and inducers present in the cell. But glucose will be utilised even smallest amount present in the medium as it does not depend on lac operon.

Thus a mutation that keeps catabolite activator protein (CAP) from forming properly, in the cell, D. Lactose would accumulate, glucose concentration would decrease.

Thanks for asking.