Specific interaction of histone H1 with eukaryotic DNA

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. © 1981 IRL Press Limited.