LEC24: An Example of Regulated Txn & mRNA Processing I Flashcards
what is normal hemaglobin structure?
2 alpha, 2 beta chains
fetal vs adult globin chains?
fetus: gamma chain
birth: switch to beta chain
where are alpha and beta globin genes?
1 beta gene on chromosome 11
2 alpha genes on chromsome 16
what turns on beta globin expression? where is it?
KLF1, txn factor
binding sites close to beta clobin gene (-90) and in the LCR, locus control region
when is KLF off/on?
KLF off in progenitor and proerthroblast; turned on in erythroblast stage; get beta globin gene expression once KLF1 synthesis turned on
how does beta globin gene expression get turned on?
1) KLF1 gene on chromosome 19 turned on in erythropoiesis program at progenitor cell stage
2) KLF1 modified post-translationally by Phosphorylation at T41
activates KLF1
3) KLF1 interacts w/ CBP, a large complex including a protein w/ HAT activity
4) this increased acetylation opens up chromatin in vicinity; get txn of B globin gene
what is structure of fetal hemoglobin?
when does this change?
2 alpha & 2 gamma chains
2 alpha globin genes on chromosome 16; 2 gamma globin genes on chromosome 11, pustream of B globin gene
several weeks before birth, level of gamma globin chains begins to decrease; elvel of beta globin chains increases
trend continues until few months after birth, when beta gene expression is max, gamma is low
what is the adult form of hemoglobin?
HbA, alpha-2-beta-2
HbF constitutes <1% of HbA
what turns on BCL11A?
what is BCL11A?
KLF1 turns on BCL11A; recall, KLF1 turns on beta globin
BCL11A is a txn repressor that turns off the gamma globin gene
SO same txn factors turns on beta and represses gamma
what does KLF1 do?
activates B-globin txn and inactivates gamma-globin expression
what do HbS sickle cell disease patients express re: globin chains?
normal Beta chains replaced w/ BetaS
major hemoglobin form = HbS, a2BS2
what is hereditary persistance of fetal hemoglobin (HPFH)
condition in which gamma chains are expressed at significant level into adulthood
limits tendency of HbS to polyermize
pts w/ >15% of HbF show few disease signs; also beneficial in pts w/ B-thalassemia, who have poor B globin expression
for whom is HPFH beneficial?
patients w/ B-thalassemia, who have poor expression of B globin
what is the mutation associated w/ HPFH, hereditary persistence of fetal hemoglobin?
BCL11A enhancer mutation
intron 2 mutation!
less BCL11A expression –> more gamma expression
what are the steps of pre-mRNA processing?
1) 5’ capping
2) splicing
3) 3’ Poly A
what is the 5’CAP made of?
what is the bond it makes?
what does it enable?
CAP = 7-methyl-guanisine
linked to 5’ end of transcript in 5’ to 5’ phosphodiester bond
needed for translation initiation by RNA Pol II
what is the 5’ CAP function?
only mRNAs
1) recognition site for binding of proteins that recruit a ribosome
2) protects pre-mRNA and mRNA from degradation by exonucleases that chew in 5’ to 3’ direction
what does intron begin and end w/ usually?
begin: GU
end: AG
what is the branch point of an intron?
where a 2’ to 5’ phosphodiester bond is formed when intron is spliced out
site of the 1st transesterification rxn
what does spliceosome do?
huge complex of 200 proteins
has class of noncoding RNAs, SnRnPs - small nuclear ribonucleoprotein particles
catalyzes accurate splicing
what is proces of splicing?
1) 2’OH of branch point A attachs the G of the loose consensus sequence at the 5’ splice site
2) 3’ OH of the upstream exon is now epxposed at 5’ splice site; it attacks phosphate of the phosphodiester bond, btwn end of intron and beginning of downstream exon, at 3’ splice site
**lariat RNA **forms during process of splicing
what would happen if splicing is incorrect by 1 ntd?
frameshift mutation downstream
where is the poly A signal?
what does it signal for?
what binds it?
where is poly A signal?
what does it do?
how is it added on?
at end of 3’ UTR sequence
signals for endonucleolytic cleavage 10-30 ntds downstream
free 3’ hydroxyl is acted on by Poly-A polymerase, which adds 100-200 A residues in a template-indepenent fashion
what is poly(A) binding protein?
binds poly(A) tail
