nucleosome

nucleosome. The nucleosome is the fundamental repeating unit of eukaryotic chromatin, first described by Roger Kornberg in 1974. It consists of approximately 147 base pairs of DNA wrapped around a core of eight histone proteins, forming a structure that compacts the genome and regulates access to the genetic code. This organization is essential for DNA packaging, transcription, DNA replication, and DNA repair.

Structure and composition

The canonical nucleosome core particle is composed of a histone octamer, containing two copies each of the core histones H2A, H2B, H3, and H4. This protein complex forms a disc-shaped structure around which double-stranded DNA is wound in approximately 1.65 left-handed superhelical turns. The assembly of this structure is facilitated by histone chaperones like Nucleoplasmin and Chromatin Assembly Factor-1. Linker DNA connects adjacent nucleosomes and is associated with the linker histone H1, which promotes higher-order chromatin folding into the 30-nm fiber. Key structural insights have been derived from X-ray crystallography studies, notably those conducted at the MRC Laboratory of Molecular Biology.

Histone modifications and variants

Histone proteins undergo extensive post-translational modifications that alter chromatin structure and function. Common modifications include acetylation, methylation, phosphorylation, and ubiquitination, catalyzed by enzymes such as histone acetyltransferases and histone deacetylases. The bromodomain and chromodomain are protein modules that recognize specific modifications. Additionally, non-allelic histone variants, like H2A.Z, H3.3, and CENP-A, are incorporated into nucleosomes at specific genomic locations, influencing processes such as centromere function and DNA damage response. The Polycomb group proteins and Trithorax group proteins are key regulators maintaining these epigenetic states.

Nucleosome positioning and dynamics

Nucleosomes are not uniformly distributed along the genome but are positioned by a combination of DNA sequence preferences, ATP-dependent chromatin remodeling complexes, and transcription factor binding. Remodelers like SWI/SNF, ISWI, and INO80 complexes use ATP hydrolysis to slide, evict, or restructure nucleosomes. Nucleosome-free regions often coincide with promoter sequences, such as those for the RNA polymerase II machinery, allowing transcription factor access. Techniques like MNase-seq and chemical mapping have revealed positioning patterns around functional elements like transcription start sites and enhancer regions across various organisms from Saccharomyces cerevisiae to Homo sapiens.

Role in gene regulation

Nucleosomes act as dynamic gatekeepers of gene expression. Repressive chromatin states, marked by modifications like H3K9me3 and associated with proteins like HP1, can silence genes, while active states marked by H3K4me3 or H3K27ac promote transcription. The preinitiation complex and general transcription factors like TFIID must often contend with nucleosomal barriers. Pioneering transcription factors, such as those in the FOXA family, can bind nucleosomal DNA and initiate remodeling. Nucleosome occupancy and positioning directly influence the recruitment of the Mediator complex and the progression of RNA polymerase during transcription elongation.

Experimental methods and research

The study of nucleosomes relies on a suite of biochemical and genomic techniques. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) maps histone modifications and transcription factor binding sites genome-wide. Micrococcal nuclease digestion is a classic method for nucleosome mapping. Advanced structural methods, including cryo-electron microscopy at facilities like the European Synchrotron Radiation Facility, have revealed nucleosome dynamics. Seminal research from institutions like the Cold Spring Harbor Laboratory and scientists including David Allis and Michael Grunstein has elucidated the roles of histone modifications and chromatin in gene regulation and disease states such as cancer.

Category:Molecular biology Category:Chromatin Category:Epigenetics