Histones and nucleosomes

The miraculous way in which long chromosomal DNA is packed into the cell nucleus is still not fully explained. Eor human chromosomes the required reduction in length is by a factor of around 1(F. 

The chromosomal DNA in eukaryotic cells appears as thin chromatin fibres which consist of about 60% protein, 35% DNA and probably 5% RNA. The chromatin fibres are folded and looped into bundles in which the DNA is associated with the protein. The protein units are known as 

The amino acid sequences of each of the live varieties of histones, taken from several animal species, have been determined. The histones have an exceptionally high content of basic amino acids, particularly lysine and/or arginine and this gives them a positive charge at neutral pH (isoelectronic point 10.8). 

Electrostatic attraction between the positively charged amino acids, and the negatively charged phosphate groups in the DNA chains probably accounts for much of the binding of the latter to the histones. The histones are known to be bound to specific regions of the DNA and are unevenly distributed along the DNA chains. 

Since the histones themselves are phosphorylated, there may be competition with the DNA for binding to the positively charged regions of the histone proteins. The removal of OH in the phosphorylation process may also affect the H bonding system and thence the secondary and tertiary histone structure. 

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Do you think that our knowledge of histones and nucleosomes is nearly complete ... 

Much of the DNA is associated with histone proteins to form a structure called the nucleosome. Nucleo-somes are composed of an octamer of histones and 150 bp of DNA. 

Some of this differential expression is achieved by having different regions of chromatin available for transcription in cells from various tissues. For example, the DNA containing the P-globin gene cluster is in active chromatin in the reticulocyte but in inactive chromatin in muscle cells. All the factors involved in the determination of active chromatin have not been elucidated. The presence of nucleosomes and of complexes of histones and DNA (see Chapter 36) certainly provides a barrier against the ready association of transcription fac-... 

Fig. 10.2. FSPIM analysis of the interaction between maize transcriptional coactivators—GCN5 and ADA2—fused to CFP and YFP. GCN5 is a histone acetyltransferase that, in conjunction with adaptor protein ADA2, modulates transcription in diverse eukaryotes by affecting the acetylation status of the core histones in nucleosomes [63]. CFP- and YFP-tagged proteins, expressed in protoplasts, were excited by the 458 nm and the 514 nm laser lines sequentially. CFP fluorescence was selectively detected by an FIFT 458 dichroic mirror and BP 470-500 band pass emission filter while YFP fluorescence was selectively detected by using an HFT 514 dichroic mirror and...

However, the packing of DNA into nucleosome-like structures is not unique to eukarya similar structures appear in archaea (reviewed in Reeve et al., 1997). Additionally, histones and minichromosome maintenance proteins (MCM) are widespread among eukarya and archaea and absent in prokarya, and the eukaryotic chromo domain has a structure that is highly reminiscent of archaeal histones that are involved in formation of archaeal chromatin (Ball et al., 1997). Consequently, it is possible that chromatin remodeling in eukaryotes is an elaboration of a similar cellular mechanism in archaea. 

This may be correlated with the conservation of the primary sequences of the carboxy-terminal regions of the histones and the indications that these are the sites of histone-histone interaction within the nucleosomes (Spiker and Isenberg, 1978). 

During nucleosome reconstitution, performed by mixing core histones and DNA in 2 M NaCl with slow back-dialysis to low salt con-... 

There is a strong electrostatic contribution to the interaction of histones with DNA. Increasing concentration of salt induces the stepwise dissociation of histones from chromatin and nucleosomes. There are three stages in this process. In the first stage, up to 0.7 M NaCl, HI is selectively dissociated. Between 0.7 and 1.2 M NaCl, H2A and H2B are removed, whereas the arginine-rich histones H3 and H4 are dissociated only above 1.2 M NaCl. At 2.0 M NaCl the DNA of chromatin is free of histones (Ohlenbusch et al., 1967 Burton et al., 1975). 

The fact that nucleosome-like particles can be reversibly reconstituted from histones and DNA, without any additional factor, exemplifies the principle of self-assembly of biological structures. The reconstituted particles have the characteristic beaded appearance of nu-... 

Furthermore, there is a striking parallelism between these data and the neutron diffraction data from nucleosomes in 100% D 0 (Pardon et al., 1977 Suau et al., 1977), where scattering from the histone protein dominates, and from core protein in 2 M NaCl solution (Pardon et al., 1978). The above interference phenomenon may well be the explanation for the protein-dominated scattering maximum between 35 and 37 A observed for chromatin and nucleosomes in solution (Pardon et al., 1977 Suau et al., 1977). 

Linker histones (HI, H5 and others) are also major components of metaphase chromosome, and occupy 5.8% of the total protein amount (Uchiyama et al, 2005). They play an important role in the formation of the 30 nm fiber (see also section 2.3). These linker histones carry more lysine residues ( 30% of the total amino acids) than the core histones and have a core domain in the middle part that binds to a nucleosome. The linker histones could be easily extracted from the chromatin with 0.5 M NaCl, whereas the core histone octamers need more than 0.8 M NaCl to dissociate from nucleosomes. 

Peterson CL, Laniel MA (2004) Histones and histone modifications. Curr Biol 14, R546-551 Pfaffle P, Gerlach V, Bunzel L, Jackson V (1990) In vitro evidence that transcription-induced stress causes nucleosome dissolution and regeneration. J Biol Chem 265 16830-16840 Poch O, Winsor B (1997) Who s who among the Saccharomyces cerevisiae actin-related proteins ... 

Akey CW, Luger K (2003) Histone chaperones and nucleosome assembly. Curr Opin Struct Biol 13 6-14 Aoyagi S, Hayes JJ (2002) hSWI/SNF-catalyzed nucleosome sliding does not occur solely via a twist-diffusion mechanism. Mol Cell Biol 22 7484—7490... 

DNA is packaged in the nucleus into the form of chromatin. Chromatin is a nucleoprotein complex composed of histone and non-histone proteins, DNA and RNA and it exhibits a repeating structure (van Holde, 1988). The basal unit of chromatin, the nucleosome, is composed of a histone octamer (two each of H2A, H2B, H3 and H4) around which two superhelical turns of DNA are wrapped (van Holde, 1988). The structure of both the histone octamer (Arents et al, 1991)... 

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