Genet 302 lec 37 & 38
What problem does dosage compensation solve in Drosophila?
Males have only one X chromosome, so without compensation they would make too little X-linked gene product
Why is too little X-linked gene product a problem?
Proteins must be produced in balanced amounts, especially when they interact with autosomal proteins
What is the Drosophila solution to dosage compensation?
Increase X-linked gene expression in males
What are MSL complexes made of?
Proteins and RNAs
What protein components of the MSL complex were emphasized?
MSL1, MSL2, MSL3, and MOF
What RNA components of the MSL complex were emphasized?
Rox1 and Rox2 RNAs
Why were MSL proteins discovered in mutagenesis screens?
Because mutants lacking them showed a male-specific lethal phenotype
Why are MSL complexes only made in males?
Because MSL2 is turned on in males but off in females
How does the sis gene affect MSL2?
sis indirectly regulates the MSL2 gene
What is the MSL2 state in wild-type XY males?
On
What is the MSL2 state in wild-type XX females?
Off
What is the MSL2 state in XO sterile males?
On
What is the MSL2 state in XXY fertile females?
Off
How do MSL complexes first bind the X chromosome?
They are recruited to about 200 chromatin entry sites, or CES sites, found only on the X chromosome
What happens after MSL complexes bind CES sites?
They spread laterally along the X chromosome
Where are MSL complexes especially enriched after spreading?
At active gene bodies on the X chromosome
What protein is necessary for lateral spread of the MSL complex?
MSL3
What do MSL complexes do on the X chromosome?
They increase gene expression
What histone modification covers much of the male X chromosome?
H4K16ac
What enzyme in the MSL complex makes H4K16ac?
MOF
What kind of enzyme is MOF?
A histone acetyltransferase
How does H4K16ac increase gene activity?
It interferes with 30 nm chromatin fiber formation and makes chromatin more accessible
Which hypothesis best explains how H4K16ac acts in Drosophila dosage compensation?
The accessibility hypothesis
What is the overall mechanism of dosage compensation in Drosophila?
MSL complexes form only in males, bind the X chromosome, spread along it, acetylate H4K16, and increase X-linked gene expression
What is the simple dosage-compensation solution in birds?
Increase Z-linked gene expression in females
What is the complex dosage-compensation solution in mammals?
Increase X-linked gene expression in both sexes, then inactivate one X in females
What problem does mammalian dosage compensation solve?
XX females would otherwise make too much X-linked gene product compared with XY males
What is step 1 of mammalian dosage compensation?
Count the X chromosomes
When are X chromosomes counted in human embryos according to the lecture?
About 2 weeks old
What is step 2 of mammalian dosage compensation?
If there are two X chromosomes, turn off one
What is step 3 of mammalian dosage compensation?
Maintain the inactive X epigenetically
What is the inactive X chromosome called?
Xi
What is the active X chromosome called?
Xa
What is a Barr body?
An inactive X chromosome in an interphase nucleus
How many Barr bodies are in a 46,XX female?
1
How many Barr bodies are in a 46,XY male?
0
How many Barr bodies are in a 45,X female with Turner syndrome?
0
How many Barr bodies are in a 47,XXY male with Klinefelter syndrome?
1
How many Barr bodies are in a 47,XXX female?
2
What DNA region is important for counting X chromosomes?
The X inactivation centre, or Xic
What is Xist?
A long functional RNA, not an mRNA
What does Xist stand for?
X-inactive specific transcript
About how long is Xist RNA?
About 17,000 nt
What is a Xist complex made of?
Xist RNA plus proteins
Which X chromosome does the Xist complex bind?
Usually a single X chromosome that will become Xi
How does a Xist complex spread on the X chromosome?
It forms at the Xic and then moves laterally along that chromosome
Why are Xist complexes usually only made in females?
Because cells with two X chromosomes usually silence Xist on one X and allow Xist to accumulate on the other
What RNA opposes Xist?
Tsix
What is Tsix?
A long functional RNA transcribed opposite to Xist
About how long is Tsix RNA?
About 40,000 nt
What does Tsix do?
It prevents synthesis of Xist RNA
How can Tsix prevent Xist synthesis according to the lecture?
By masking or by transcriptional interference
Why is only one Tsix gene usually active per XX cell?
Because there are only enough positive transcription factors for one active Tsix gene per cell
What happens on the X chromosome with active Tsix?
It remains the active X, Xa
What happens on the X chromosome where Tsix is silent?
Xist accumulates and that chromosome becomes Xi
What do Xist complexes do on the X chromosome they bind?
They mark it as inactive, then epigenetic mechanisms repress gene expression on that X
What epigenetic changes decrease on Xi?
Activating marks such as H3K4me and various H3 and H4 acetylation
What is one example of an activating mark lacking on Xi?
H3K4me
What chromosome regions are not inactivated on Xi?
Pseudoautosomal regions
Why does dosage compensation usually not act on female germline cells?
So eggs will always receive an active X