Operon Theory is the concept of gene regulation proposed by François Jacob and Jacques Monod (1961). An operon is a group of structural genes whose expression is coordinated by an operator. The repressor encoded by a regulatory gene binds to the operator and represses the transcription of operon. In the presence of inducer, the repressor is inactivated and dissociates from operator to express the operon. Thus, the expression of the operon is controlled by a cis-acting operator and by a trans-acting repressor.
In prokaryotes, many functionally related structural genes involving catabolite metabolism or biosynthetic pathways are arranged adjacently on the genome to form an operon. The genes are transcribed as a single mRNA from a promoter which is located at the 5′ end of the first gene. Therefore, operon is a transcription unit for the gene expression. Operon theory is the model of how to switch on and off the structural