The Concept of Lac Operon

In 1961, Jacob and Monad performed a number of experiments to find out the nature of induction of enzyme synthesis in E.coli. Of the 800 enzymes thought to be synthesized by the bacteria, some enzymes are synthesized continuously and are called CONSTITUTIVE ENZYMES; others are synthesized only in the presence of an inducer compound, which may not be the substrate and are called INDUCIBLE ENZYMES.

E.coli can grow tremendously on a culture medium containing glucose. However, when bacteria transferred to a medium containing lactose and not glucose, it will not grow immediately, but after a short span of time, it begins to show the same growth rate as seen on a glucose medium. Investigators have revealed that growth on the lactose medium required the presence of two substances not normally synthesized: Beta-galactosidase (which hydrolyses lactose to glucose and galactose), and lactose permease (which enables the cell to take up lactose).

LAC OPERON

The lac operon has to do with the ability of E.coli to utilize lactose sugar. Lactose is a disaccharide, which is made up of glucose and galactose. 

The enzyme present in E.coli that breaks down lactose is Beta-galactosidase

The lac operon is actively a series of adjacent genes and regulatory elements in one small part of the E.coli circular chromosome. 

Lac Operon Component:

 

I = Inducer/regulator gene – this gene is transcribed to make mRNA and then translated to a repressor protein or allosteric protein, which can either activate or suppress the operator gene. 

O = Operator – a short sequence of bases that acts like a switch that can be recognized by repressor protein.

Operon = a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. 

ZYA = structural genes

Z – Codes for β-galactosidase

Y –Codes for lactose permease, a protein that functions to actively bring lactose from outside the cell to the inside, even against a concentration gradient.

A – Codes for transacetylase, an enzyme that is also needed to breakdown many sugars related to lactose.

ð  All three genes that code for enzymes needed to use β-galactosidase molecules as a source of carbon and energy are adjacent to each other and are co-ordinately turned on or off. 

ð  Operons are found only in prokaryotes. In eukaryote, each gene has its own promoter and regulatory elements.

Mechanism of Lac Operon:-

Step: The promoter for the I gene is always ON, but is very weak so it is transcribed only rarely.

-          The I-mRNA is translated into a polypeptide -- 4 copies make one repressor protein. A typical cell will have only about 10 copies of this protein.

-          In the absence of lactose, the repressor protein binds to the operator preventing transcription from the second promoter. Almost no ZYA mRNA is made.

CASE I: When only lactose is present, the model works as follows –

When the concentration of lactose in the cell increases, it means, there is shortage of glucose. In such a condition, lactose binds to the repressor protein and inactive it so that the protein no longer could bind to the operator. When the repressor protein is not bound to the operator, Z, Y, and A genes are transcribed and translated. 

CASE II: When lactose is not present, the model works as follows:

When lactose is not present in the cell, it means, there is sufficient quantity of glucose in the cell. In such a condition, no further glucose synthesis is required. So, the repressor protein binds to the operator and blocks it. Therefore, no ZYA genes are transcribed or translated.

Note: In the first case when lactose is high in the cell, it promotes the genes to transcribe and translate. Therefore, lactose acts as an inducer. 

CASE III: When both glucose and lactose are present

If both glucose and lactose are present, cells use up the glucose before turning on the Lac Operon. When energy begins to become limiting, a signal moledule, cyclic adenosine mono phosphate (cAMP) builds up; it binds to a catabolic activating protein (CAP) and the complex in turn binds to a site between the promoter segments of the Lac Operon.

Binding of the cAMP/CAP complex opens the promoter for RNA polymerase binding.

Note:

-          When glucose level is high – cAMP is low

-          When glucose level is low – cAMP is high