7.3 The eukaryotic chromosome (continued)
Nucleosomal DNA packaging into a 30 nm fibre: the role of histone H1
When chromatin is isolated from the nucleus and examined under the electron microscope, it can be seen as a 30 nm fibre. This fibre is formed through the action of the histone H1 on the nucleosomal DNA in the 10 nm fibre. In contrast to the other histone proteins, H1 does not contain the histone fold motif.
Compaction of the 10 nm fibre to give the 30 nm fibre is achieved by interaction of the H1 protein with both the linker DNA and the histone octamers, as shown in Figure 31. The H1 protein is rich in lysine residues, which are able to neutralise the negatively charged backbone. The C-terminal tail of H1, which interacts with the linker DNA regions between the nucleosomes, is critical in this respect. The lysine-rich N-terminal tails of the octamer histone proteins also make a major contribution to neutralising the negative charge of the DNA backbone and facilitating compaction. As well as compacting the DNA, the pulling together of nucleosomes in this way shields both the exposed linker DNA regions and the exposed DNA faces on nucleosomal DNA. Thus H1-mediated compaction serves to further regulate access to DNA.
The genes encoding H1 histones are evolutionarily less well conserved than those that encode the core histones, but similar lysine-rich, positively charged proteins are found in organisms as diverse as fungi and protoctists, indicating that the role of the ‘linker’ histone is well conserved. There are also many variant forms of H1 and in several cases it is replaced by specialist proteins or other molecules. One example of packaging being assisted by another molecule is the formation of highly compacted chromatin in sperm where a variant H1 combines with positively charged molecules called protamines to package DNA.