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Chapter 13
11-10-04
Prokaryotic Gene Regulation
Bacteria place a premium on being energy efficient.
Prefer to use food sources that are easiest to "digest" & transcribe genes (synthesize
mRNA) & translate proteins only when those proteins are needed.
Ex: Sugar utilization.
Glucose is the preferred carbon source because it goes directly into the glycolysis
pathway.
Any other sugar must be converted to glucose before it can enter the glycolysis pathway
& this takes energy.
It is advantageous to have the potential to utilize other sugars (carbon sources), but
want to only synthesize the proteins necessary for utilization of these sugars
only when glucose is absent & these specific sugars are present.
This is accomplished by regulating the transcription of genes that code for proteins
necessary for utilization of other sugars, such as lactose & arabinose.
Transcription is regulated by proteins that bind to DNA.
Positive regulation:
Activator protein binds to operator regions that are usually in between promoter
(where RNA Pol. binds) & txn. start site. (Some activators bind promoter
regions).
Binding of activator facilitates txn; without presence of activator, RNA Pol may
bind to the promoter, but much less efficiently.
Negative regulation:
Repressor protein binds to operator & blocks txn of downstream genes.
Binding of Activator or Repressor proteins is regulated allosteric effector molecules.
One part of the protein binds to DNA = DNA binding site (or DNA binding
domain).
Sequence-specific binding  can only bind to specific DNA sequence that is
present in specific operator region.
Another part of the protein binds to a specific allosteric effector.
Binding of effector to the allosteric site changes the conformation of the DNA binding
domain, changing the ability of the protein to bind to DNA.
In some cases, effector binding is necessary in order for the protein to bind to
DNA.
In other cases, effector binding prevents protein from binding to DNA.
Lactose (Lac) Operon
Single promoter (P) = binding site for RNA Pol.
3 structural genes that are transcribed  multigenic mRNA:
lac Z  codes for -galactosidase.
-galactosidase catalyzes the breakdone of lactose into galactose & glucose.
lac Y  codes for permease.
Permease transports lactose into the cell.
lac A  codes for transacetylase that isn't involved in lactose metabolism.
Regulation of lac operon
1. Negative regulation.
lac I gene (close to rest of lac operon, but that doesn’t matter).
Constitutively transcribed, codes for lac Repressor protein.
In absence of lactose, lac Repressor binds to lac Operator & blocks txn of Z, Y, &
A genes.
In presence of lactose, which is the allosteric effector for the lac Repressor, lactose
binds to the allosteric site of lac Repressor & prevents it from binding to DNA.
Relief of repression = Induction.
Allosteric effectors that cause such relief of repression are called inducers.
When lactose is not present, it would be very wasteful to expend the energy to
synthesize the lac mRNA & translate the lacZ & lacY genes.
2. Positive regulation.
Catabolite repression.
This is the mechanism by which operons that code for proteins necessary for
metabolism of other sugars are only allowed to be transcribed when
glucose (the preferred carbon source) is absent.
CAP = Catabolite Activator Protein
CAP binds to the CAP site in the lac promoter.
Binding of CAP causes DNA to bend, making it easier for RNA Pol to
bind to the promoter.
In order for CAP to bind to the promoter, its allosteric site must bind to
cAMP.
Adenylate cyclase is an enzyme that catalyzes the synthesis of cAMP from
ATP.
This enzyme is inhibited by glucose.
Therefore, when level of glucose is high, cAMP concentration is low
& CAP is unable to bind to DNA & txn of lac operon is inhibited.
Only when level of glucose is low, cAMP concentration is high
& CAP-cAMP able to bind to lac promoter, facilitating txn of lac operon.
Therefore, full txn of lac operon only happens when:
Lactose is present  relieves repression by binding to lac Repressor.
&
Glucose is absent  activates CAP by increasing cAMP levels.