Class Notes (808,126)
Canada (493,084)
Biology (Sci) (2,417)
BIOL 200 (478)
Richard Roy (213)

22. Chromatin Modification.pdf

6 Pages
Unlock Document

McGill University
Biology (Sci)
BIOL 200
Richard Roy

looped domain looped domainNaveen Sooknanan McGill Fall 2011 Chromatin Modification: As we have learned before, DNA does not exist “naked” in the cell. It undergoes extensive wrapping to form chromatin in the mitotic phase  Through associations with histones 1-4, DNA is wrapped into higher order nucleosomes  These nucleosomes can undergo further folding into 30nm fibers, 300nm fibers, 700nm fibers and eventually 1400nm chromatin fibers While DNA is normally found in its wrapped up form, it must be unwound in order for protein factors involved in transcription to bind to the DNA and carry out transcription  Modification in chromatin structure severely affects gene expression (transcription) levels  Unwinding of the chromatin facilitates binding of transcription factors thus increasing the rate of transcription  This unwinding can be carried out by a series of proteins and complexes, including histone modifying enzymes, chromatin remodelling complexes and components of RNA Polymerase II Discovery of chromatin unwinding began with work on Saccharomyces cerevisiae (baker’s yeast)  Baker’s yeast exists in two states: a diploid state (2 sets of chromosomes) and a haploid state (1 set of chromosomes) o Yeast likes the haploid state and spends most of its time like this  During the haploid phase, one yeast cell can be either an a or α mating type o The mating type is determined by three genetic loci, positions on the chromosome (chromosome III to be specific), and their expression  HMLα and HMLa are two mating type loci located on the telomeric ends of the chromosome o These genes must be silenced (i.e. must not be transcribed) otherwise the cell would be diploid and would not mate  The MAT locust (mating locust) is located in the middle of the chromosome and is the only one expressed  By complex endonucleic activity, HMLα and HMLa are brought to the MAT region where they can be expressed  The cell can only mate when it is in haploid state and it contains α and a in its mating locust  Certain sequences of the yeast chromosome around the HML loci contain specific silencer sequences which somehow code for the transcriptional repression of these regions Transcriptional repression through silencer sequences is not limited to mating loci; it can be seen in the blocking of expression of tRNA genes (done by Pol III) as well as telomeric regions of other chromosomes 1lc) SIR3 protein (al Repressor-directed histone deacetylation lc) SIR3 protein (al Repressor-directed histone deacetylationNaveen Sooknanan McGill Fall 2011  We now know that histone wrapping plays a huge role in transcriptional repression through interaction of various factors with these silencer sequences The factors involved in activating the silencing process of the mating type loci were identified through genetic screening  RAP1 is a protein involved in binding to specific silencer sequences as well as repetitive sequences found on telomeric regions of the chromosome  SIR1 (silent information regulator 1) cooperates with RAP1 by binding to it and allowing other factors to recognize the resulting complex  SIR3 and 4 then recognize this complex and bind to the silencing region, causing recruitment of another factor  SIR2 then binds to SIR4 and carries out a deacetylase function on the histone tails which are attached to the DNA o Histone tails normally carry a net positive charge and are thus capable of interacting with the phosphate group of the DNA backbone, thus causing this wrapping effect o While acetylated DNA is loose because it carries a neutral charge, SIR2 takes the removes these acetyl groups, which re-wraps the DNA/histone structure by renewing this positive charge on the histone tails  These histone tails are not hypoacetylated (less acetyl groups)  SIR3 and 4 can recognize these hypoacetylated tails and form even larger complexes by recruiting more SIR2 and causing further de-acetylation o This spreads the wrapping effect all over the sequence to be silenced, effectively stopping any transcription from happening  This method effectively shuts down various regions of the chromosome containing silencer sequences or telomeric repeats  SIR3 has the specific function of co-localizing with telomeric DNA o When the cell is stained in such a way as to only reveal the telomeres, it can be seen that SIR3 proteins are capable of binding to these telomeric repeat regions through certain kinds of microscopy Transcriptional repressors (opposite to transcriptional activators we have discussed before) are DNA binding factors which may act through histone deacetylation complexes (HDACs)  As stated before, histone tails (particularly the N terminal tails) are positively charges and are capable of electrostatically interacting with the phosphate on the backbone of the DNA  When this happens, the DNA wraps up and transcription factors such as TBP are unable to bind to the TATA box o Thus the pre-initiation complex in transcription cannot be formed on hypoacetylated regions of the chromosome 2Euchromatin (active/open) AC Me Euchromatin (active/open) AC MeNaveen Sooknanan McGill Fall 2011  Acetylation of these histone tails neutralizes the charge and releases the histone/DNA complex, allowing these TFs to interact and transcribe properly  Taking again the yeast chromosome, it is possible for specific genes to be repressed by the binding of protein complexes capable of carrying out a deacetylase function o URS1 is the gene that needs to be repressed in this case o Ume6p (specific to this gene) the binds to DNA binding protein (DBP) already bound to a region upstream of URS1 and calls in another protein called Sin3 o Sin3 interacts with a protein called Rpd3 which has a deacetylase function on histone tails o Rpd3 then deacetylase regions around this URS gene, hypoacetylating these regions and wrapping the TAT box and other nearby regions up into histones  This DNA binding protein is known as a co-repressor and effectively stops transcription of many genes within the chromosome by calling in appropriate deacetylation machinery Transcriptional activators, on the other hand, usually have an acetylation which carry out the exact opposite function than the one described above  Almost the same sequence occurs, but with different proteins  Gcn4 binds to a DBP attached to a sequence upstream of a gene needing to be activated (in this case, UAS)  Gcn5, part of a multisubunit complex involving CBP and p300, the binds to this complex on the UAS gene and carries out an acetylation reaction
More Less

Related notes for BIOL 200

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.