Post transcriptional regulation- Exam 3

mechanisms of posttranscriptional gene regulation

control of alternative splicing

mRNA stability

translation

RNA silencing

proteome vs genome

genome: complete sequence of an organism’s genetic material (DNA): it is about 3 billion base pairs

approx. 20,000 protein coding regions have been identified

proteome: complete set of proteins an organism can produce; estimated about 100,000 produced; due to alternative splicing of mRNA transcripts

alternative splicing

generates different forms of mRNA from identical pre-mRNA (increases number of proteins)

expression of one gene gives rise to numerous proteins with similar and different functions

increases number of proteins made from one gene: # of proteins cell can make is not directly related to number of genes in genome

at least 2/3s of protein-coding genes in humans undergo this

prior to leaving the nucleus the introns are removed and exons exit the nucleus

introns

alternative splicing can produce different polypeptides from the same gene region because of these

may make it easier to produce proteins with different domains

they may be relics of ancient viruses

may aid in recombination

calcitonin

1 gene region that produces two distinctly different polypeptides

32 amino acid polypeptide that regulates calcium levels in the blood during childhood

splicing of exons 1,2,3, 4 create calcitonin

calcitonin gene related peptide (CGRP) regulates calcium levels in the brain and the peripheral nervous system: splicing of 1,2,3,5,6 create this

Dscam Gene

found in drosophila which produces a protein that guides the growth of axons

contains 4 exons that are highly variable with alternative forms that can be combined into over 38,000 unique polypeptides (isoforms)

these polypeptides guide the connections that develop among axons

cells express unique combinations of isoforms

each individual isoform will bind only to the same isoform enabling specific connections

Human homolog of Dscam

Dscam: abbreviation for Down’s syndrome cell adhesion molecule

in humans it is thought that the Dscam gene product codes for transmembrane protein that promotes cell-cell interactions

it is expressed at very high levels in fetal tissue

overexpression is seen in individuals with Down’s syndrome

not clear if the human homolog undergoes alternative splicing to the degree in drosophila

spliceopathies

mutations that affect regulation of splicing and contribute to several genetic disorders

ex: myotonic dystrophy

control of mRNA stability

steady-state level of mRNA: amount of mRNA in cell available for translation: determined by combination of transcription and mRNA degradation rates

half-life: mRNA is degraded at some point after synthesis, lifetime of mRNA varies; regulated by cell need

Three pathways of degradation

1. enzymes shorten length of poly-A tail :binding of poly-A binding protein to tail stabilizes mRNA

2. decapping enzymes removes 7-methylguanine gap- mRNA is now unstable

3. endonuclease cleave mRNA internally

proteosome

cylindrical structure that recycles amino acids

ubiquitin

a protein that tags proteins for recycling by the proteosome

translational and posttranslational regulation

p53 protein: if levels increase, the cell suffers DNA damage or metabolic stress → activates Chk2 ATM and ATR which adds PO4 to p53 protecting it from Mdm2

p53 is a transcription factor that induces transcription of Mdm2 gene (ubiquitin ligase, blocks transcription): adds ubiquitin to p53 send to proteosome

ubiquitin tags proteins (p53) for degradation by enzymes

lots of signals activate p53 and in turn it activates many downstream processes

cell survival signals activate Mdm2 allowing it to degrade p53

if PO4S are bound to N terminus of p53 Mdm2 cannot bind and p53 can continue to activate transcription of its targets