The lac operon system - Notes of Regulation of gene expressions

 

In a Cell,  the gene are expressed to perform a particular function or perform a group of functions. Like in Body of E. coli,  an enzyme beta galactosidase is the responsible to catalyse the hydrolysis of a disaccharide lactose into glucose and galactose.

Lactose is source of energy in the body of the E coli. if E coli do not get  the lactose around them as a source of energy. Then it is not capable to synthesis the enzyme beta galactosidase for a long time therefore it is the metabolic physiological or environmental condition that regulate the expression of gene. 

Generally there are two types of gene regulation. First in the positive and second is  negative regulation. In Negative regulation, the gene continue to express their effect till their activity is suppressed. This  type of regulation is also called as repressible regulation. The positive regulation is the one in which the gene remain non Express unless they are induced to do it. It is therefore called inducible regulation. 

In Eukaryotes the regulation of gene expression takes place at the various level. These level are - transcription level, processing level, transport of RNA  from nucleus  to the cytoplasm and translational level. 

In prokaryotes, the initiation process of transcription is the predominant site for the control of gene expression. In a transcription unit the activity of RNA polymerase is regulated by interaction with the accessory protein.

The accessibility of promoter region of prokaryotic DNA is in the many cases regulated by the interaction of protein with the DNA sequences called operator. The operator region is located near  to the promoter region in most of operon system and in most cases the sequence of the operator binds with the repressor protein. 

Each operon system has its specific operator and specific repressor region. 

The lac operon system

The Francois Jacob and Jacque Monad were the first to describe a gene regulatory system at the transcriptional level. This system was called the Lac Operon System. This system can be seen in the body of the E coli. In Lac Operon lac is referred to as a lactose or allolactose. 

In the body  of E. coli or In a lac operon, a Polycistronic,  structural gene is regulated by a common promoter and the regulatory gene.

The Lac Operon consists of one regulatory gene which is denoted by i and There are three structural gene z y and a. The  I gene code for the repressor of of lac Operon. The z gene code beta-galactosidase enzyme which is the primary responsible for the hydrolysis of lactose into glucose and galactose.The gene y  code for the permease enzyme  which increase the permeability of cell of E.  coli for beta galactosidase. The y gene is responsible for transacylase enzyme.The function of the transacylase enzyme is unknown so far. All three enzymes are needed in the Lac Operon for the metabolism of the lactose.

Lactose is the substrate for the enzyme beta galactosidase and it is responsible for the switch on or off for operon system therefore lactose act as inducer. 

When lactose is absent

if lactose is not present near the E coli then the repressor protein is formed by regulatory i gene through the process of translation from repressor mRNA, this  repressor protein binds to the operator region and prevent RNA polymerase  to proceed the transcription. This situation is called operating system is switched off. 

When lactose is present

In presence of lactose or in presence of inducer repressor protein that is formed from  represor mRNA, and get inactivated and it is not capable to bind the operator region.  As a result the structural  gene containing z y a gene form lac mRNA through transcription. And finally all the protein beta-galactosidase permease  and transacetylase are formed through the process of the translation. Therefore this condition is operating system is switched on.