Lac Operon and Mode of Regulation

Hi there

E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide.

However, lactose needs to be hydrolysed (digested) first. So the bacterium prefers to use glucose when it can.

Four situations are possible:

When glucose is present and lactose is absent the E. coli does not produce galactosidase.

When glucose is present and lactose is present the E. coli does not produce galactosidase.

When glucose is absent and lactose is absent the E. coli does not produce galactosidase.

When glucose is absent and lactose is present the E. coli does produce galactosidase.

The control of the lac operon

1. When lactose is absent

A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lac operon, the Operator site. The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing.



2. When lactose is present

A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site).



This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site.



This explains how the lac operon is transcribed only when lactose is present.

BUT….. this does not explain why the operon is not transcribed when both glucose and lactose are present.

3. When both glucose and lactose are present

When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and it keeps falling off.



4. When glucose is absent and lactose is present

Another protein is needed, an activator protein. This stabilises RNA polymerase.
The activator protein only works when glucose is absent.



In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose.