Biology THE Lac OPERON


`star` Introduction
`star` Components of Lac Operon
`star` Regulation


● The elucidation of the lac operon was also a result of a `color{Violet}"close association"` between a geneticist, `color{brown}"Francois Jacob"` and a biochemist, `color{brown}"Jacque Monod"`.

● They were the first to elucidate a `color{Violet}"transcriptionally regulated system"`.

● In `color{Violet}"lac operon"` (here lac referes to `color{Violet}"lactose"`), a `color{Violet}"polycistronic"` structural gene is regulated by a `color{Violet}"common promoter"` and `color{Violet}"regulatory genes"`.

● Such arrangement is very common in `color{Violet}"bacteria"` and is referred to as `color{Violet}"operon"`.

● To name few such examples, `color{Violet}"lac operon"`, `color{Violet}"trp operon"`, `color{Violet}"ara operon"`, `color{Violet}"his operon"`, `color{Violet}"val operon"`, etc.


● The lac operon consists of `color{Violet}"one regulatory gene"` (the `color{Violet}"i gene"` – here the term i does not refer to `color{Violet}"inducer"`, rather it is derived from the word `color{Violet}"inhibitor"`) and three `color{Violet}"structural"` genes (`color{brown}"z, y, and a"`).

● The `color{brown}"i gene"` codes for the `color{Violet}"repressor"` of the lac operon.

● The `color{brown}"z gene"` codes for `color{Violet}"beta-galactosidase"` (`color{Violet}"β-gal"`), which is primarily responsible for the `color{Violet}"hydrolysis"` of the disaccharide, `color{Violet}"lactose"` into its monomeric units, `color{Violet}"galactose and glucose"`.

● The `color{brown}"y gene"` codes for `color{Violet}"permease"`, which increases permeability of the cell to `color{Violet}"β-galactosides"`.

● The `color{brown}"a gene"` encodes a `color{Violet}"transacetylase"`.

● Hence, all the `color{Violet}"three gene products"` in lac operon are required for metabolism of lactose.

● In most other operons as well, the genes present in the operon are `color{Violet}"needed together"` to function in the same or `color{Violet}"related metabolic pathway"`.


● `color{Violet}"Lactose"` is the substrate for the enzyme `color{Violet}"beta-galactosidase"` and it regulates `color{Violet}"switching on and off"` of the operon.

● Hence, it is termed as `color{Violet}"inducer"`.

● In the `color{Violet}"absence"` of a preferred carbon source such as `color{Violet}"glucose"`, if lactose is provided in the growth medium of the bacteria, the `color{Violet}"lactose"` is transported into the cells through the `color{Violet}"action of permease"`.

`star` (Remember, a very `color{Violet}"low level of expression"` of lac operon has to be present in the cell all the time, otherwise `color{Violet}"lactose cannot enter"` the cells).

● The lactose then `color{Violet}"induces the operon"` in the following manner.

● The `color{Violet}"repressor"` of the operon is `color{Violet}"synthesised"` (all-the-time – `color{Violet}"constitutively"`) from the `color{Violet}"i gene"`.

● The repressor protein `color{Violet}"binds to the operator"` region of the operon and `color{Violet}"prevents RNA polymerase"` from transcribing the operon.

● In the presence of an inducer, such as `color{Violet}"lactose or allolactose"`, the repressor is `color{Violet}"inactivated"` by `color{Violet}"interaction"` with the inducer.

● This allows `color{Violet}"RNA polymerase access"` to the `color{Violet}"promoter"` and transcription proceeds.

● Essentially, regulation of lac operon can also be visualised as `color{Violet}"regulation of enzyme synthesis"` by its substrate.

● Regulation of `color{Violet}"lac operon"` by repressor is referred to as `color{Violet}"negative regulation"`.

● Lac operon is under control of `color{Violet}"positive regulation"` as well, but it is beyond the scope of discussion at this level.