THE lac OPERON

Each cell in the human contains all the genetic material for the growth and development of a human. Current estimates suggest this consists of 25000 to 30000 genes.

Some of these genes will be need to be expressed all the time. These are the genes that are involved in of vital biochemical processes such as respiration.

Other genes are not expressed all the time. They are switched on an off at need.

Operons

An operon is a group of genes that are transcribed at the same time. They usually control an important biochemical process. They are only found in prokaryotes.

The lac operon was the first to be discovered by François Jacob and Jacques Monod in the 1959.

It was the first example of gene control to be worked out.

The lac operon consists of three genes each involved in processing the sugar lactose. One of them is the gene for the enzyme galactosidase. This enzyme hydrolyses lactose into glucose and galactose.

Using strains of the bacterium Escherichia coli (E. coli), which had different mutations, they were able to discover the way in which this bacterium managed its sugar metabolism.

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.

Summary

© Paul Billiet 2008