Chapter 22 - The Posttranscriptional Regulation of Gene Expression In Eukaryotes

What are the two levels of post-transcriptional regulation in eukaryotes?

Nuclear post transcriptional regulation and Cytoplasmic post transcriptional regulation

How does post-transcriptional regulation help a cell respond to changes?

It allows the cell to respond to changes quicker

What role does post-transcriptional regulation play during development?

It helps the cell properly express proteins during development

How does post-transcriptional regulation affect protein production?

It increases the number of proteins from a limited number of genes

Where does transcription occur in eukaryotic cells?

In the nucleus

What happens to mRNA before it exits the nucleus?

It is heavily modified

What is the result of modifying mRNA from a single gene?

More than one protein product can be produced

What is alternative splicing?

The process that can remove introns or exons from mRNA

What does alternative splicing create?

A new mature mRNA

Is alternative splicing more common in higher eukaryotes or single-celled eukaryotes?

Higher eukaryotes

What percentage of mammalian genes are affected by alternative splicing?

More than 95%

What directs alternative splicing?

ESE/ISE and ESS/ISS

How is Drosophila sex determined?

By the ratio of X chromosomes to autosomes (A).

What is the X to autosome ratio for a female Drosophila?

1 (2X:2A)

What is the X to autosome ratio for a male Drosophila?

0.5 (1X:2A)

What are the activators on the X chromosome in Drosophila?

sis-a and sis-b

What is the repressor found on chromosome 2 (A) in Drosophila?

dpn

What do the activators sis-a and sis-b bind to during early Drosophila development?

A promoter upstream of the sxl gene.

What are the two phases of sxl gene regulation in Drosophila?

Pe (establishment) and Pm (maintenance).

What happens to Pe and Pm in Drosophila sexual differentiation?

Pe is turned off and Pm is activated in both males and females.

What does early Sxl protein bind to in Drosophila?

Early Sxl protein binds to an ISS at the 3' end of the intron between exons 2 and 3.

What is the effect of early Sxl protein binding in Drosophila?

It blocks the proper association of U2 and U2AF, producing late Sxl protein.

What role does late Sxl protein play in Drosophila?

It acts as a splicing inhibitor on the sxl gene and reinforces its own expression.

What do male Drosophila lack in terms of Sxl protein?

Male flies do not produce early Sxl protein.

What is the result of the lack of early Sxl protein in male Drosophila?

The protein produced from the sxl gene includes exon 3 with a stop codon, resulting in a truncated, non-functioning Sxl protein.

What role does late Sxl protein play in Drosophila sexual differentiation?

Late Sxl protein regulates transcription of the transformer (tra) gene.

What is the function of Tra protein in Drosophila?

Tra protein is a splicing activator that binds to an exonic splicing enhancer (ESE).

How does late Sxl protein affect Tra protein production in females?

In females, late Sxl protein acts as a splicing inhibitor, leading to the skipping of exon 2 of tra and producing functional Tra protein.

What is the outcome of male transcription of the tra gene?

Male transcription of the tra gene produces an mRNA that includes exon 2 and its stop codon, resulting in a non-functional Tra protein.

What regulates the alternate processing of dsx gene pre-mRNA in Drosophila sexual differentiation?

Tra protein

What proteins direct splicing to the 3' end of exon 4 in Drosophila?

Tra, Tra2, and Rbp1 (SR proteins)

What happens to exon 4 in female Drosophila during sexual differentiation?

It is clipped off, producing a short, female-specific version of the Dsx protein.

What happens to exon 4 in male Drosophila during sexual differentiation?

It is skipped, producing a longer, male-specific version of the Dsx protein.

Why do Tra2 and Rbp1 not bind to pre-mRNA in male Drosophila?

Because Tra protein is not produced in males.

What is the N-terminus of Dsx in both male and female Drosophila?

They have the same N-terminus.

What is the role of the male Dsx C-terminus?

It is an inhibitor of female genes.

What is the role of the female Dsx C-terminus?

It is a strong activator for female genes.

What is a characteristic of pre-mRNA regarding cleavage sites?

A pre-mRNA may have multiple 3'-cleavage and polyadenylation sites.

What percentage of human genes have multiple alternative 3'-end cleavage sites?

As many as 60% of the genes in humans have multiple alternative 3'-end cleavage sites.

What is a well-studied example of a gene with multiple cleavage sites?

The heavy chain of IgM.

What type of cells express the IgM gene?

B cells.

What happens to the 3'-cleavage site when IgM is on the surface of the B cell?

The chosen 3'-cleavage site has hydrophobic amino acids at the end.

What happens to the 3'-cleavage site when IgM is made into an antibody?

A different 3'-cleavage site is chosen, making it hydrophilic.

What is the role of translational control in Eukaryotes?

Translational control is responsible for the cell's fate.

What type of genes does translational control affect?

It controls the expression of long genes.

At which levels can regulation occur in gene expression?

Regulation may occur at both transcriptional and translational levels.

Why is translational regulation important during development?

It is needed to create a local gradient of the protein's level.

What type of cells do not have transcriptional control?

Non-nucleated cells do not have transcriptional control, only translational control.

How can initiation factors regulate translation?

Initiation factors can be phosphorylated by kinases.

How can proteins act as repressors in translational regulation?

Proteins may bind directly to mRNA at the 3' UTR and act as a repressor.

What is one way proteins can disrupt translation initiation?

Proteins can disrupt the interaction between eIF4E and eIF4G.

What is the most common type of regulation in eukaryotic initiation?

Phosphorylation of initiation factors

How can phosphorylation of initiation factors affect protein binding to the 5' UTR?

It can be inhibitory or stimulatory

What happens during the inhibition of translation involving eIF2α?

Phosphorylation of eIF2α leads to interaction with eIF2B, resulting in no translation

What is the effect of phosphorylation of eIF4E-binding proteins (4E-BP)?

It leads to the release of eIF-4E and formation of the 40S complex

What is the role of reticulocytes in hemoglobin production?

Reticulocytes make hemoglobin, which has 2α and 2β polypeptide chains (globins).

What happens to globin translation without heme?

Translation of globins is blocked.

What is the function of the kinase heme-controlled repressor (HCR)?

HCR phosphorylates eIF2α.

What does phosphorylated eIF2α bind to?

Phosphorylated eIF2α binds strongly to eIF2B.

What is the role of eIF2B in translation initiation?

eIF2B exchanges GTP for GDP.

What is the consequence of tight binding of phosphorylated eIF2α to eIF2B?

No release occurs, preventing the exchange of GDP for GTP.

What is the result of having no GTP in the translation initiation process?

No attachment of tRNAiMet to 40S, leading to no initiation of translation.

What does eIF4E bind to?

The 5' cap

What does eIF4E interact with?

eIF4G

What domain on eIF4G interacts with eIF4E?

The same domain is also found in 4E-BP

What does 4E-BP do in relation to eIF4E?

It competes with eIF4G for eIF4E

What is the effect of unphosphorylated 4E-BP on translation?

It binds tightly to eIF4E and inhibits translation

What happens when 4E-BP is phosphorylated?

It does not bind to eIF4E, allowing eIF4E to bind to eIF4G and promote translation

Which kinase is responsible for the phosphorylation of 4E-BP?

mTor

What activates mTor?

Growth factors, hormones, and more

What is the consequence of overexpression of eIF4E?

It can lead to cancer

What is the effect of inhibiting mTor with rapamycin?

No phosphorylation of 4E-BP, leading to its binding to eIF4E and inhibition of translation

What do upstream open reading frames (uORF) do?

They do not produce a functional protein.

How do uORFs regulate translation?

By diverting the ribosomes.

What is an example of a gene that contains uORFs?

The GCN4 gene in yeast.

How many uORFs are present before the Gcn4-coding sequence in the GCN4 gene?

Four uORFs.

What does the regulation of translation by uORFs depend on?

The phosphorylation of eIF2α.

What effect do low nutrient conditions have on translation initiation at the start site?

It prevents the ribosome from starting translation.

What occurs in response to amino acid starvation in yeast?

Low levels of eIF2α phosphorylation

What does phosphorylated eIF2α induce the expression of?

The global transcription factor Gcn4

How many different genes is Gcn4 in charge of for biosynthesis?

40 different genes

What happens to ribosomes when eIF2α is unphosphorylated?

Ribosomes bind to the uORF and are less likely to bind to the start AUG of GCN4

What happens to ribosomes when eIF2α is phosphorylated?

Ribosomes are less likely to start translation at uORF and will scan the mRNA for a longer period of time to reach the AUG

What is the role of the 3' UTR of mRNA in translation initiation?

It communicates with the 5' end via protein-protein interactions.

What is the purpose of the interaction between the 3' UTR and the 5' end of mRNA?

It helps ensure that the mRNA is fully processed before it leaves the nucleus.

Which proteins can repress translation of certain mRNA by interacting with eIF4G?

Proteins that interact with eIF4G, including Maskin.

What is the function of the protein Maskin in translation regulation?

It interacts with CPEB bound to the CPE and inhibits other proteins from interacting with eIF4E.

During what developmental stage is the regulation of translation initiation particularly important?

During embryo development.

What is the relationship between translation and mRNA degradation?

Translation and degradation of mRNA are inversely correlated.

When are mRNAs stable in the cytoplasm?

mRNAs are stable in the cytoplasm if actively being translated.

When are mRNAs unstable in the cytoplasm?

mRNAs are unstable in the cytoplasm if poorly translated.

What is one factor that affects mRNA degradation?

The removal of the 3' poly(A) tail.

What is another factor that affects mRNA degradation?

The removal of the 5' cap by the formation of macromolecular assemblies.

What are processing bodies (P bodies)?

Cytoplasmic structures that form in the cytoplasm of a cell after starvation.

What do processing bodies (P bodies) contain?

They include the mRNAs being destroyed.

How can eukaryotic cells regulate a large number of genes at once?

Eukaryotic cells can regulate a large number of genes at once via multiple mechanisms.

What is one mechanism of gene regulation in eukaryotic cells?

Regulating pre-mRNA splicing in the nucleus.

How is pre-mRNA splicing regulated in eukaryotic cells?

Pre-mRNA splicing is regulated by changes in nutrient availability.

What role do the 5' UTR and 3' UTR play in gene regulation?

Physical changes to the UTR can lead to repressing translation or stabilizing the mRNA.

What are AU-rich elements (AREs) in mRNA?

AU-rich elements are sequences that target mRNA for rapid degradation.