Regulation of Prokaryotic Gene Expression

PROKARYOTIC REGULATION

• Bacteria that can conserve resources and energy  have an advantage over those bacteria that cannot.  

• Natural selection has favored those that can only produce genes/products when needed by the cell.

There are two key metabolic pathways:

1. The building of a molecule/anabolic pathway

   1. only done when levels of the molecule become too low

   2.  relies on feedback inhibition -- when there is enough of the product, it will inhibit further production 

2. The breakdown of a molecule/catabolic pathway

   1. Only done when a molecule is present and needs to be broken down

PARTS OF AN OPERON

Operon -- the entire stretch of DNA required for enzyme production and comprised of several parts

• Operator -- a segment of DNA that controls the access of RNA polymerase to the genes

• Repressor -- binds to the operator to block access to a gene

• Regulatory Gene -- produces the promoter

• Promoter -- where the RNA polymerase binds in order to transcribe the structural genes

REPRESSIBLE OPERON

These genes are part of an anabolic pathway.

The product can shut down its own production when enough is available.

The product inhibits further production

TRP OPERON

The TRP operon is a repressible operon.  In the absence of tryptophan, RNA polymerase can access the structural genes and produce tryptophan.  

When enough tryptophan is present, it ACTIVATES the repressor and it BLOCKS further production of tryptophan.

Here, tryptophan is the COREPRESSOR.  It is a molecule that binds to the repressor and ACTIVATES it halting further production.  It’s like a co-captain.  Both parts are needed to repress further production.

TRP OPERON -- this is a repressible operon.  

• Transcription is usually ON and and will be REPRESSED when the COREPRESSOR (tryptophan) binds with the REPRESSOR.  

• The REPRESSOR is produced INACTIVE. 

  • When activated, it RESPRESSES the operon.

      

INDUCIBLE OPERON

An inducible operon requires an INDUCER to induce transcription.

The active form of the repressor is when EMPTY/when it is first produced.

A molecule (the inducer) needs to bind to the repressor and inactivate it, causing transcription to be induced

LAC OPERON

These genes are part of a  CATABOLIC PATHWAY -- lactase, the enzyme that breaks down lactose, is only produced when lactose is present.  

Because we only want to produce these genes when a particular molecule is present, the repressor is ACTIVE upon production.  We want this USUALLY OFF and only on when needed.

Here, lactose/allolactose is the INDUCER. 

 When present, it binds to the repressor and inactivates it thus inducing transcription.

REPRESSIBLE vs INDUCIBLE OPERON

Both REPRESSIBLE and INDUCIBLE operons are an example of NEGATIVE GENE REGULATION

The ACTIVE form of the REPRESSOR BLOCKS TRANSCRIPTION

• A repressible operon requires a corepressor to activate the repressor and block transcription; it is active when bound to the corepressor

• An inducible operon has an active repressor and requires an inducer to induce transcription (deactivate the repressor); it is active when empty/not bound to the inducer

POSITIVE GENE REGULATION

When a protein binds to the promoter, it speeds up the rate of transcription.

It is not an ON/OFF switch, it’s more of a volume knob.

The lac operon/repressor model is the ON/OFF switch.  The addition of this other protein to the promoter allows RNA polymerase to bind more efficiently (like another 10 sets of hands to grab RNA polymerase)

LAC OPERON POSITIVE CONTROL

Two Molecules involved:

1. cAMP -- cyclic AMP -- which is a molecule that accumulates when glucose is scarce (Glucose↓ so cAMP ↑)

2. Regulatory protein - CRP --  (cAMP receptor protein)  is the activator; a protein that binds to DNA and stimulates transcription. (sometimes CRP is called CAP)

LAC OPERON

Under dual control: 

1. Negative control -- the presence of lactose induces lactase production; it is regulated by the repressor at the operator of the operon

2. Positive control -- as glucose levels decrease, cAMP increases, binds with CRP which, in turn, binds to the promoter thus TURNS UP THE VOLUME of lactase production.

   1. If the amount of glucose increases, cAMP decreases and dissociates from CRP.  This causes the operon to still be ON, but transcription is very slow.