Operons are segments of genetic material which function as regulatory units or units or units that can be switched on and off. Operon concept was given by two French scientists, working at Pasteur Institute, Francois Jacob and Jacques Monod (1961). While studying lactose utilisation in mutants of E. coli.
An operon consists of one to several structural genes (three in lac operon and five in tryptophan operon of E. coli, nine in histidine or operon of Salmonella typhimurium), one operator gene, a promoter gene, a regulator gene, a repressor and inducer/or corepressor. Operons are of two types, inducible and repressible.
Inducible Operon System/Lac Operon System: An inducible operon system is that regulated genetic material which remains regulated genetic material which remains switched off normally but becomes operational in the presence of an inducer. It occurs in catabolic pathways. The components are:
(1) Structural Genes:
They are genes which produce m RNAs for forming polypeptide/proteins/ enzymes. Lac operon of E. coli has three structural genes-Z (produces enzyme p-galactosidase for splitting lactose/galactoside into glucose and galactose), Y (produces enzymes galactoside permease required in entry of lactose/galactoside) and A. (.produces transacetylase without any function in E. coli). The three structural genes of lac operon produce a single polycistronic m RNA. The three enzymes are, however, produced in different concentrations.
2. Operator Gene:
It gives passage to RNA polymerase when the structural genes are to express themselves. Normally it is covered by a repressor. Operator gene of the lac operon is small, made up only of 27 base pairs.
3. Promoter Gene:
It is recognition centre/ initiation point for RNA polymerase of the operon.
4. Regulator Gene (i Gene):
It produces a repressor that binds to operator gene for keeping it nonfunctional to pass from promoter to structural genes).
It is a small protein formed by regulator gene which binds to operator gene and blocks passage of RNA-polymerase towards structural enzymes. Repressor has two allosteric sites, one for attaching to operator gene and second for binding to inducer. Repressor to lac operon has a molecular weight of 160,000 and 4 subunits of 40,000 each.
It is a chemical which attaches to repressor, changes the shape of operator binding site so that repressor no more remains attached to operator. Lactose/galactose is inducer of lac operon. As soon as the operator gene becomes free, RNA polymerase is recognised by promoter gene. This process requires c AMP. RNA polymerase passes over the operator gene and then reaches the area of structural genes. Here it catalyses transcription of m RNAs.