2. Learning Objectives

2.7. Chromatin remodeling

It is the dynamic modification of chromatin architecture to allow access of condensed genomic DNA to the regulatory transcription machinery proteins, and thereby control gene expression.
•When chromatin is tightly packed, and not actively being transcribed it is called heterochromatin.
•When chromatin is more loosely packed, and therefore accessible for transcription it is called euchromatin.
•Chromatin remodeling is highly implicated in epigenetics.
•Epigenetic modifications to histone proteins such as methylation/demethylation and acetylation/deacetylation can alter the structure of chromatin resulting in transcriptional activation or repression.

Two classes of chromatin remodeling enzymes
a) Class I : Histone modifying enzymes
  • These do not alter nucleosome position
  •  They bring about covalent modification of histone proteins like  histone tail modifications (Ac, Me, P, Ub, etc.)
  •  Proteins recruited by these modifications include: i)transcription factors ii)ATP-dependent nucleosomal remodeling enzymes iii)histone modifying enzymes

 

b) Class II : Chromatin remodeling factors
•It shifts nucleosome position with respect to DNA, exposing regulatory sequences.
•These are often referred to as Swi/Snf factors (because they were first identified as yeast mutants defective in mating type switching and in the ability to metabolize sucrose , sucrose non-fermenting).
•Chromatin remodeling factors use energy from ATP hydrolysis to rearrange the packing of nucleosomes in higher order chromatin structures.
•Remodeling improves access to DNA or histone binding sites recognized by transcriptional regulators or histone modifiers.
• Some of these bind to :
    •  Activation domains and de-condense the associated chromatin.
    • Repression domains and condense the associated chromatin.