H4 histones

Spt6 H3-H4 Histone-transfer vehicle Transcription-elongation factor (FACT)... 

Histones are highly basic, small, compact proteins, with a high affinity for DNA. They occur naturally, attached to the DNA of cell nuclei by ionic linkages. Their classification is based on the relative amounts of lysine and arginine. The galactosylated, lysine-rich histone HI was found to be superior to the H2-H4 histones as a DNA carrier for liver gene delivery (53). 

Found in the chromatin of all eukaryotic cells, histones have molecular weights between 11,000 and 21,000 and are very rich in the basic amino acids arginine and lysine (together these make up about one-fourth of the amino acid residues). All eukaryotic cells have five major classes of histones, differing in molecular weight and amino acid composition (Table 24-3). The H3 histones are nearly identical in amino acid sequence in all eukaryotes, as are the H4 histones, suggesting strict conservation of their functions. For example, only 2 of 102 amino acid residues differ between the H4 histone molecules of peas and cows, and only 8 differ between the H4 histones of humans and yeast. Histones HI, H2A, and H2B show less sequence similarity among eukaryotic species. 

A nucleosome core particle is formed by a 146 base pairs DNA fragment wrapped 1.65 times around the histone octamer (formed by two copies of H2A, H2B, H3 and H4 histones). NCP are connected by regions of naked DNA called linkers. The nucleotide sequences of the 146 bp DNA fragments involved in nucleosomes are polymorphic. The regions of contact between DNA and the core of histones are distributed along DNA with a periodicity of around 10.4 bp, which is the number of base pairs per helical turn of DNA in the nucleosome. 

Figure 10-4. Schematic representation of nucleosomes showing DNA wrapped around a core of histones H2A, H2B, H3, and H4. Histone HI associates with the linker region of DNA.

Describe the composition of chromatin. List the types of histones and describe their general characteristics. Note the evolutionary stability of the sequences of the H3 and H4 histones and assign the types of histones to their locations within the nucleosome. 

There are five separate histone isoforms histones HI, H2A, H2B, H3, and H4. Histones H2A, H2B, H3, and H4 assemble as dimers to form an octamer (2 x 2A -i- 2 x 2B -i- 3 -i- 4 = 8). The DNA wraps 1.75 turns around this octamer. Histone HI acts as a between the octameric units. When viewed ruider the electron microscope, the packaging yields a stracture that is reminiscent of a string of beads in which octamers and associated DNA are the beads and the linker is the HI boruid to DNA. A bead and its linker are referred to as a nucleosome. 

HATB HAT1 Yeast-human H4 Histone deposition... 

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