MCB 250 – Chromatin Remodeling and DNA Methylation

Module 30

how is chromatin structure regulated?

• modification of histones associated with the DNA

• methylation of DNA itself

what is chromatin remodeling?

a process in which nucleosomes are either moved or disassembled to expose the blocked sequence(s)

what does Gal4 do when galactose is present?

Gal4 activates transcription of a set of genes required for galactose metabolism

1. cis-regulatory region of GAL1 gene contains four UAS sequences (UAS_g enhancer)

   • UAS_g enhancer contains RSC chromatin remodeling complex → ensures that enhancer DNA is partially unwound → leaves Gal4 binding sites accessible

2. Gal4 homodimer binds to at least one UAS (upstream activating sequence)

3. activation domain of Gal4 recruits SWI/SNF chromatin remodeling complex

4. SWI/SNF removes the nucleosome covering the core promoter and +1 start site of GAL1

5. GAL1 transcription activates

how can chromatin structure be remodeled?

modification of amino acid R groups on N-terminal tail domains of core histones:

• acetylation

• phosphorylation

• methylation

main purpose of acetylation and phosphorylation of histone tails

loosens DNA → euchromatin → increases rate of transcription

most common form of modification

histone acetylation → by histone acetyltransferases (HATs)

• deacetylation by histone deacetylases (HDACs)

what type of motifs do acetylated histone tails recruit?

bromodomains → increases accessibility of DNA for transcription

main purpose of methylation of histone tails

compacts DNA → heterochromatin → decreases rate of transcription

what type of motifs do methylated histone tails recruit?

chromodomains → silences transcription of surrounding DNA

main target for DNA methylation

CpG sequence (“p” stands for phosphodiester bond)

how does DNA methylation regulate chromatin structure and silence gene transcription?

1. DNA methyltransferase methylates both strands of CpG sequence by adding methyl groups to C5 of cytosine rings

2. methylation of multiple sites within a CpG island results in gene silencing

what are CpG islands?

clusters of CpG sites

what does epigenetic refer to?

persistent states of gene regulation → information passed along generations without modification in the DNA sequence

what is maintenance methyltransferase and what does it do?

an enzyme that restores hemimethylated CpGs into fully methylated CpGs