Gene Expression and Protein Targeting

Flashcards for review of lecture notes on gene expression and protein targeting.

What determines the amount of protein produced per mRNA?

The biological half-life of the mRNA

What is a polyribosome?

One mRNA being simultaneously translated by many ribosomes

Name four types of posttranslational modifications.

Folding, covalent attachments, S-S bridge, proteolytic cleavage, multi-subunit association

Where does all protein synthesis initiate?

On free, unbound ribosomes in the cytosol.

What determines whether a ribosome is free or membrane bound?

The protein being synthesized.

How do proteins get to the right place in a cell?

Proteins have amino acid signals that direct them.

Name four compartments that proteins are targeted to.

Cytosol, Nucleus, Mitochondria, Chloroplasts, Peroxisomes, Endoplasmic Reticulum, Nuclear Envelope, Golgi, Lysosomes

What is the Nuclear Localization Sequence (NLS)?

A small sequence of C-terminal amino acids common on some nuclear proteins that acts as a targeting signal.

Where do proteins destined for the Endomembrane System or for secretion begin their synthesis?

On free ribosomes, then they move to the RER membrane.

Define mutation.

A change in genetic sequence.

What are the three levels at which genes can be controlled?

Transcriptionally, translationally, or post-translationally.

What is constitutive expression?

When some genes are always actively expressed.

What are the levels of gene control?

Transcriptional, translational, and post-translational control.

What is an operon?

A cluster of genes that are coordinately regulated by one promoter.

What is the operator?

A short DNA sequence that controls access to the promoter by RNA Polymerase.

What are the two general types of operons?

Negatively regulated and positively regulated.

How does a repressible operon work?

It is normally active but is turned off when a specific Repressor protein binds to the operator.

How does a positively regulated operon work?

The genes are normally off, and binding of an Activator protein to the DNA turns transcription on.

How does the trp Operon work?

The repressor protein, trp Repressor is the product of a gene (trpR) located outside and upstream of the operon.

What is an allosteric protein?

Proteins that exist in two distinct conformations: active conformation and inactive conformation.

How does an inducible operon work?

It is normally off because of an active repressor, but the operon can be turned on in the presence of an inducer that inactivates the repressor.

What is the function of -Galactosidase (-Gal)?

Cleavage of the β- glycosidic bond in lactose.

What does the lac operon comprise?

A promoter, an operator, and 3 genes.

How does the lac operon work when lactose is absent?

The lac repressor binds to the operator, preventing RNA polymerase from binding to the promoter and moving forward.

How does the lac operon work when lactose is present?

Some lactose is converted to allolactose, which binds to and inactivates the lac repressor, allowing transcription to occur.

What is the difference between the lac operon and the trp operon?

The lac operon is inducible, while the trp operon is repressible.

Why is the lac operon is inducible?

Repressor normally bound and then unbinds; Inducer converts repressor to inactive form; Operon transcribable

Why is the trp operon repressible

Repressor normally unbound and then binds; Corepressor converts repressor to active form; Operon not transcribable

What pathways do inducible operons generally function in?

Catabolic pathways

What pathways do repressible operons generally function in?

Anabolic pathways

How is glucose sensed by E. coli?

A Regulatory Protein (Catabolite Activator Protein - CAP) interacts with a small signaling molecule(Cyclic AMP (cAMP)).

How is CAP regulated?

Allosterically; cAMP binding to CAP stabilizes CAP in the active conformation.

How is the lac operon dually controlled?

Negatively by the lac repressor and positively by CAP.

When lactose is present and glucose is also present, how active is the lac operon?

Weakly active

When lactose is present and glucose is absent, how active is the lac operon?

Highly active