Operon Concept:
Francois Jacob and Jacques Monod (1961), while working at the Pasteur Institute, Paris proposed that metabolic pathways are regulated as a unit. They studied the change in the nature of enzymes of E.coil when provided with different kinds of sugars. They found when the sugar lactose is added to the cultures of E.coil, it induces three enzymes necessary to break down the lactose into glucose and galactose. These newly synthesized enzymes are-
-
i. β-galactosidase
ii. Permease
iii. Transacetylase
According to them, the synthesis of these three enzymes is controlled by a long DNA segment known as Operon which is divisible into an operator site O and three structural genes Z, Y, and A. The action of structural genes is regulated by the operator site with the help of a repressor protein produced by the action of gene ‘i’ known as the regulator gene.
The genes are expressed or not expressed depending on whether the operator switch is on or off. When the switch is on, the three genes are transcribed by RNA polymerase into a single stretch of messenger RNA covering all three genes. Each structural gene is generally known as cistrons and the transcribed long mRNA covering all the cistrons is known as polycistrons.