Question

An E. coli mutant isolated is simultaneously unable to utilize a large number of sugars, e.g. lactose, arabinose, maltose, as carbon source.  However, genetic analysis shows that each of the operons responsible for metabolism of each sugar is free of mutation.  Name a specific gene the mutation of which may account for this outcome?   Explain.

Answer 1

Answer: Mutation in the CAP protein
Explanation:
E coli utilizes several sugars as carbon sources. Each operon is responsible for the uptake and catabolism of the respective sugar. However, these operons are active only in the absence of glucose.
i.e. glucose is the preferred source of carbon for energy metabolism as it can directly enter into the respiratory pathway. On the other hand, other sugars have to be converted first into a respiratory intermediate to enter the catabolic pathways.

It is given that an E coli mutant is isolated that is unable to utilize any of the sugars as carbon sources. However, none of the operons exhibit any mutations. Each of these operons senses the presence of glucose by CAP protein. The CAP protein activates an operon only when the glucose levels are low. So, the operon is not activated/transcribed if the CAP protein is mutated.