Question

In your practical class you discover a new histone variant, H3-NEW, that contains more aspartate and glutamate amino acids than the common core histone H3.

1. Your practical partner suggests that promoters where core histone H3-NEW is bound are likely to be less active than promoters where core histone H3 is bound. Is your practical partner correct? Explain your reasoning. [6 min]
2. Describe how you would experimentally determine whether H3-NEW exhibits a different dynamic exchange rate (association/dissociation) with the nucleosome than the core histone H3. [10 min]
3. What factors may regulate the incorporation of this novel histone variant into a nucleosome? [4 min]

Answer 1

Histones are positively charged proteins which get bound with DNA due to electrostatic attraction between positively charged amino acids in proteins and negatively charged phosphates . If a histone in question is mutated and contain more negatively charged amino acids like aspartate and Glutamate then interaction of this new histone H3-NEW will be affected with other histones in CORE of nucleosome . This CORE of nucleosome is made of two H2A , two H2B, two H3 and two H4 histones and H1 provide stability to nucleosome by binding with linker DNA .

If mutation occurs in histone leading to change in net charge then interaction with proteins involved in gene regulation like HAT histone acetyl transferase and HDAC will be remarkably affected and further it will affect chromatin remodeling . These enzymes modify N terminal lysine residues and causes unwinding or decondensation of chromosome and promote gene expression .

Deacetylation of histones will reverse this and cause condensation of chromatin back to chromosomal structure and inhibit gene expression .

Nucleosome formation depends on charges on histone proteins and acetylation will increase neutrality on histones by masking its positive charges .HDAC enzyme will do the reverse .

In case of H3 N terminal residues are mostly lysine which if replaced by negatively charged amino acids like Glu, Asp then this will lead to loosening of nucleosome structure and gene expression will be affected . HAT enzymes can modify only positively charged amino acid residues not negatively charged residues so their regulation will also be affected. Overall conformation of histone will change and its interaction with other molecules too leading to instability in nucleosomal structure .

Chaperon proteins and Ubiquitin mediate destruction of mutant histones and don't allow them to be used in chromatin assembly process