H1 and H5
H1 histone. This is the most common linker histone. It helps stabilize DNA that travels inbetween the nucleosome or core histone. Although they have obtained x-ray crystallography shots of it, as seen below, there have so far been no successful attempts at obtaining a structure with DNA. It is known that it does bind DNA, obviously, but there is some controversy on where it binds DNA. A recently published paper by Mammoon, et al. showed that, at least in bacteriophages that the H1 histone binds in the major groove. It was also shown that it binds with a strong specificity, with a binding constant of 10^-8 M.
The H5 histone. This histone is more uncommon. Like the H1 histone, no one has been successful at obtaining a x-ray crystallography structure of the H5 histone with DNA. Little is known about it. What is known is that it has been found present in some cells that are transcriptionally inactive. Its function is assumed to be similar to the H1 histone. It has been found in the corresponding linker DNA inbetween nucleosomes. Although it is not completely understood, one could hypothesize that the H5 histone is only present in cells that should not trnscribe.
To learn more about the H1 protein seen above click here to get the H1 pdb file from the Protein Data Bank.
To learn more about the H5 protein seen above click here to get the H5 pdb file from the Protein Data Bank.