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Lac Operon: Negatively Controlled Inducible Operon [Gene Regulation in Prokaryotes]

Lac Operon – Negatively Controlled Inducible Operon

Lac operon is an operon required for the transport and metabolism of lactose in E. coli and some other enteric bacteria. Francois Jacob and Jacques Monod studied gene expression and formulated an operon model.

It consist of 3 adjacent structural genes, a promoter, an operator, a regulator and a terminator. It is regulated by several factors including the availability of glucose and lactose. (An operon is cluster of genes together under a single operon)

It is a negatively controlled inducible operon of gene regulation in prokaryotes because here binding of repressor inhibits transcription.

In simple terms, lac operon is off when lactose is absent and is switched on when lactose is present in the environment.

Structural genes: codes for enzymes. Structural gene of an operon usually lie adjacent to one another and RNA polymerase moves from one structural gene to the next, transcribing all of the genes into a single mRNA which is translated into enzymes.

This operon has three structure genes: lacA, lacY and lacZ

Promoter (lacP) : is the site where RNA polymerase binds to DNA to begin transcription.

Operator (lac O): resides adjacent to promoter and it serve as binding site for repressor (a gene regulatory protein which is like an off switch)

Inhibitory gene: lac i which codes for repressor protein.

00:23 introduction of operon system in prokaryotes

06:26 Lac operon in detail

12:02 Mechanism

Do you want to know what happens if glucose and lactose both are present in the environment? Check out Catabolite Repression – Positive Control of Lac Operon here.

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Bacterial Transduction – Horizontal Gene Transfer Part – III

Bacterial Transduction – Horizontal Gene Transfer Part – III

Bacterial transduction is a horizontal gene transfer method in bacteria which is mediated by bacteriophage or bacterial viruses.

It is the transfer of bacterial genes by viruses and this transfer is the result of an error made during the viral gene packaging. 

It can be of two types:

  1. Generalized transduction: It occurs during the lytic cycle (after infection of bacteria, bacteriophage take control of the host machinery and forces it to produce copies of viral components) of phages and it is called so because in this type of transduction, random fragment of degraded bacterial chromosome (genes) are packed instead of viral genes. These resulting phages are called generalized transducing particle or phages which are carrier of genetic information from donor bacteria to another cell.
  2. Specialized transduction: It occurs during the lysogenic cycle (after infection of bacteria, viral genome does not take control of its host. Instead the viral genome remains within the host cell and reproduces along with the bacterial chromosome) of phages and it is called so because in this type of transduction, a specific portion of bacterial genome gets packed due to improper excision. Therefore, along with the phage genome portion of bacterial chromosome is packed in the phage particles. When these resulting phages infect new bacteria, the previously carried bacterial genes may get incorporated into the host.

Understand the other two methods of horizontal gene transfer in bacteria here – Conjugation and Transformation.