Posted on

Catabolite Repression: Positive Control of Lac Operon [Role of cAMP & CAP]

Catabolite Repression – Positive Control of Lac Operon

Catabolite repression is a positive control of the lac operon. If bacteria have glucose and lactose both in the environment, what will it do? A smart cell will first utilise glucose and only when it is depleted it will utilize lactose. But how is it controlling lac operon even in the presence of lactose? That is where catabolite repression or positive control of lac operon comes into play.

It involves CAP protein (Catabolite Activator Protein) which is an activator protein and is also known as CRP (cAMP Receptor Protein). CAP acts as a sensor for glucose. It activates transcription of the operon but only when glucose levels are low. CAP senses glucose indirectly through the ‘hunger signal’ molecule cAMP which is a central regulator to different nutrients in environment such as glucose.

00:10 introduction

02:37 structure of the operon

04:38 role of CAP 06:21 role of cAMP

08:09 mechanism of catabolite repression

11:10 comprehensive table of all scenario of presence or absence of glucose and lactose affecting the positive control of lac operon

15:35 graphical representation

17:04 explanation of why it is called catabolite repression

Wow Tryptophan is available!!! It is party time for bacteria 😀 If you find regulation of trp operon by attenuation difficult then this video is for you 😇

Have you watch the video on lac operon? If not watch it here to understand this topic better; Lac Operon – Negatively Controlled Inducible Operon.

Posted on

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.