Specific interaction of histone H1 with eukaryotic DNA

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Abstract
The interaction of calf thymus histone H1 with homologous and heterologous DNA has been studied at different ionic strengths. It has been found that about 0.5 M NaCl histone H1, and its fragments N-H1 (residues 1-72) and C-H1 (residues 73-C terminal) , precipitate selectively a small fraction of calf thymus DNA. This selective precipitation is preserved up to very high values (>2.0) of the input histone H1/DNA ratio. The percentage of DNA in solubilized by histone Hi under these ionic conditions is dependent upon the molecular weight of the nucleic acid, diminishing from 18% for MW = 1.0 × 107 daltons to 5% for a MW = 8.0 × l04 daltons. The base composition of the precipitated DNA is similar to that of the bulk DNA. Calf thymus histone H1 also selectively precipitates a fraction of DNA from other eukaryotes (herring, trout), but not from some prokaryotes (E. coli, phage λ). On the other hand, at 0.5 M NaCl, the whole calf thymus DNA (but not E. coli DNA) presents a limited number of binding sites for histone H1, the saturation ratio histone H1 bound/total DNA being similar to that found in chromatin. A similar behaviour is observed for the histone H1 fragments, N-H1 and C-H1, which bind to DNA in complementary saturation ratios. It is suggested that in eukaryotic organisms histone H1 molecules maintain specific interactions with certain DNA sequences. A fraction of such specific complexes could act as nucleation points for the high-order levels of chromatin organization.

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