ABSTRACT
Operons are clusters of structural genes under the control of a single operator site and regulator gene which ensures that expression of the structural genes is coordinately controlled. The operon model proposes three elements: a set of structural genes, an operator site and a regulator gene. One of the most studied operons is the lac operon in E. coli. This codes for key enzymes involved in lactose metabolism: galactoside permease and ß-galactosidase. In the lac operon, the structural genes are the lacZ, lacY and lacA genes encoding ß-galactosidase, the permease and the transacetylase, respectively. High level transcription of the lac operon requires the presence of a specific activator protein called catabolite activator protein, also called cAMP receptor protein. The lac operon is a good example of negative control of gene expression in that bound repressor prevents transcription of the structural genes. Positive control of gene expression is when the regulatory protein binds to DNA and increases the rate of transcription.
