In this article we will discuss about the structure of lac operon in E. coli.
The lac operon comprises three structural genes (z, y & a) and produces a polycistronic mRNA (Lac mRNA).
This messenger RNA codes for β-galactosidase, lac permease and transacetylase. The lac permease is a single unit protein that acts as a carrier for the entrance of lactose into the cell. The trans acetylase is an enzyme of two sub-units that catalyses the transfer of one acetyl group from acetyl-CoA to galactose.
The group of structural genes is controlled by the three DNA segments, that act as regulatory elements. Three structural regulatory elements are the i+ gene or regulatory gene which codes for the lac repressor, the promoter and the operator.
The i+ gene and the repressor:
Regulator gene (i+) specifies synthesis of a repressor which due to its diffusible nature and hence acting at a distance, prevents the products of both y and z genes from being formed constitutively but not inductively. A mutation in the regulatory gene does not produce an alternation in the amino acid sequence of an enzyme, but causes a marked change in its activity.
The i gene codes for a protein repressor having 4 sub-units of 40,000 daltons each (total: 160,000 daltons). This repressor is a soluble protein that binds strongly and repressor, the lac genes will not be expressed.
Thus, the lac operon is regulated by the repressor (from the i gene), the promoter and the operator.
z, y and a genes:
z+ (cistron A) specifies the β-galactosidase (1,34,000 daltons) which gives lactose into glucose and galactose. In fact, 4 such polypeptide chains unite to form tetrameric enzyme — β-galactosidase. z+ contains 3,510 base pairs.
y+ (cistron B) does not produce catabolic enzyme but the M protein or permease at cell membrane and bacterial membrane facilitating the pumping of lactose inside bacteria, y+ contains 780 base pairs.
a+ (cistron C) specifies the thio-galactoside transacetylase, of known in vivo function but having in vitro ability for transferring acetyl group from acetyl CoA to isopropyl thiogalac- toside (IPTG) a+ contains 825 base pairs.
