• Prokaryotic Regulation

Regulation of Gene Expression – Part ISpring 2013 - Althoff Reference: Mader & Windelspecht Ch. 13)

Lec18

• Bacteria do not always ________ their entire complement of enzymes and proteins …because their environment is ever changing.

• In 1961, French microbiologists Francois Jacob and Jacques Monod showed that Escherichia coli is capable of regulating the expression of its genes.

• They observed that ________________ for a __________________ are grouped on a chromosome…and transcribed at the same time

Prokaryotic Regulation

The ___________ Model

• Jacob and Monod proposed a model to explain gene regulation in prokayotes

• An operon has these components:

1

3 4

5

2

• _______________ – normally located outside the operon. Codes for the DNA-binding protein that acts as a ______________

• _____________ - a _______________ of DNA where RNA polymerase first attaches to begin transcription (remember this happens in the nucleus) of the grouped genes.

The Operon Elements1

3

2

• __________– a short portion of DNA where an active repressor binds. When present, RNA polymerase _____________ to the promoter—thus ____ transcription possible

• _________________– genes that ______ for the enzymes and proteins that are involved in the metabolic pathway of the operon

The Operon Elements4

5

• Jacob and Monod…and others…found that some E. coli operons usually existed in the “___” condition vs. the “____” condition

• This means tryptophan is not presence and the active repressor is “inactive”

• When “___”, their products—5 different enzymes—are part of _________________ for the synthesis of the amino acid tryptophan.

• If tryptophan present in the medium, then these enzymes are not needed…and the tryptophan _____ to the repressor making it an __________________

The trp Operon: on vs. off

The trp operon – making or not making tryptophan

Tryptophan ABSENT…enzymes needed are produced

▲Tryptophan ABSENT…needed enzymes not produced

A

A

B

B

• When the trytophan binds to the repressor, it ________________…result: RNA polymerase can’t bind to the polymerase

The trp Operon: on vs. off…con’t

The lac Operon

• Bacteria metabolism is very efficient…when there is ___________ for certain proteins or enzymes, the genes needed to make them are usually inactive.

• Example: if the milk sugar lactose is not present, there is no need to “___________” genes for enzymes involve in lactose ____________.

• There are ________ encoded for the enzymes needed to break down lactose: β-galatosidase, permease, and transacetylase.

The lac Operon…con’t

• The 3 structural genes that code for producing those enzymes are ____________ to one another…

• …and are under control of a __________ promoter and a ___________ operator.

• Because the presence of lactose brings about expression of genese, it is called an ___________ of the lac operon

• The entire “unit” is called an ________________ because they have “inducible” enzymes.

Further Control of the lac Operon• E. coli preferentially breaks down

glucose…and the bacterium has a way to ensure that the lactose operon is maximally turned on ______ when glucose is absent.

• Cyclic AMP (cAMP) _____________ when glucose is absent.

• cAMP, which is a derivative of ATP, has a single phosphate group—which is attached to the ribose at two locations

• cAMP binds to a molecule called catabolite activator protein = _____…and that complex attaches to the CAP binding site next to the lac promoter.

• When CAP binds to DNA, ____________, exposing the promoter to RNA polymerase….

• …the result: RNA polymerase is now __________ to bind to the promoter aiding transcription which leads to their expression

More on cAMP…

Lactose present, glucose absent…cAMP level _______

Lactose present, glucose absent…cAMP level _____

• CAP’s protein regulation of the lac operon is an example of _________ control.

• Why? When CAP is active it ___________ the activity of the operon.

Positive Control

Negative Control • The use of repressors is an example of a _________

control• Why? When the repressor is active ____________

the operon.

Positive Controlof gene expression

Negative Control of gene expression