Question

Eucaryotes have nucleosomes that procaryotes do not. Describe how the transcriptional regulatory machinery must deal with nucleosomes. Include in your answer the roles histone modifications play in regulating transcription in eucaryotes.

Answer 1

Nucleosome is the fundamental structural subunit of chromatin, in which each of this subunit is composed of DNA that wrapped around a set of eight proteins, histone. Nucleosomes help regulate transcription because of they make certain parts of the DNA is accessible.

Eukaryotic transcriptional machinery is adapted to exploit the presence of nucleosomes in very sophisticated ways, because during the elongation of transcription, nucleosomes are acetylated and transferred behind the enzyme RNA polymerase II, in which it suppress spurious transcription initiation within the body of the gene. Nucleosomes are highly dynamic in response to the transcription process and are displaced at promoters during gene activation in the process involves ATP-dependent nucleosome remodeling complexes, modification of histones, histone chaperones, and histone variants.

Nucleosomes can slide along DNA and when the nucleosomes are spaced closely together, the transcription factors cannot bind and gene expression is turned off. When the nucleosomes are spaced far apart, the DNA is exposed and therefore, transcription factors can bind, allowing gene expression to occur.

Histone modifications: phosphorylation, acetylation, methylation, ubiquitination, citrullination, krotonilation, and isomerization may help in the regulation of transcription. For example, the acetylation (addition of acetyl group to histones) of histones alters accessibility of chromatin and this allows the DNA binding of proteins to interacting exposed sites to activate gene transcription and downstream cellular functions.