The mechanism of induction and repression were first explained by Francois Jacob and Jacques Monod in 1961, for which they were awarded Nobel Prize in Physiology and Medicinein 1965.
They proposed the lac-operon model to explain the regulation of genes coding for lactose catabolizing enzymes in E. coli. An operon is a unit of bacterial gene expression and regulation including structural genes and the control elements in the DN A recognized by the regulator gene product.
The regulator gene codes for a polypeptide known as repressor. Two repressor polypeptides associate to form afunctional repressor dimer.
Each repressor polypeptide has an active site for a molecule of lactose. Under appropriate conditions, the repressor dimer binds to another element called the operator. The regulator gene is designated as i. Some authors are of the opinion that the active repressor is a tetramer, instead of a dimer.
It is the RNA polymerase binding site present upstream of the start point of RNA transcription. The start point of protein synthesis is designated as +1. The upstream sequences are designated (-), whereas the downstream sequences (+). There are two important upstream hexamer (six nucleotide) sequences, designated as (-10) and (-35) sequences, essential for DNA unwinding and RNA polymerase binding, respectively. The (-10) sequence, rich in A and T nucleotides is referred to as the TATA box. It is also known as the Pribnow box. The promoter is symbolically denoted as
The operator is the segment of DNA lying between the promoter and the set of structural genes. The operator often overlaps the promoter. The repressor diamer / tetramer binds to this site and thus prevents the binding of the RNA polymerase and hence, transcription of the structural genes. The operator is denoted by the symbol o.
Genes which code for proteins for enzymatic and structural functions of cells are called structural genes. In E. coli, there are three structural genes, one each coding for. B-galactosidase, b-galactoside permease and b-galactoside transacetylase.
These genres are designated asz, j, and a, respectively, b-galactoside permease pumps lactose across the membrane into E. coli cells, a-galactosidase cleaves lactose into galactose and glucose molecules. The function of b-galactoside transacetylase is not yet clear. These genes are contiguously arranged and code for a single RNA molecule, which translates the above mentioned enzymes. This type of RNA is called polycistronic or polygenic RNA and the expression as coordinate expression of structural genes.