The Open Door Web Site
Molecular Genetics Index
The Chemical Nature if the Gene
The Lac Operon
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:
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.
In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose.
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