Electron microscopic study of compaction of individual DNA molecules with histone H1 in surface films

Interaction of individual spread DNA molecules with histone H1 was studied by electron microscopy using the protein-free spreading technique. It was shown that in the presence of 0.2-5 micrograms/ml of histone H1 in hypophase there are three types of structures in electron-microscopic preparations: fibres of non-compacted DNA, compact fibres with twisted strands of duplex DNA and compacted rod-like and circular structures where the separate fibres of duplex DNA could not be distinguished. The study of compact structure morphology allows us to conclude that they are formed by side-by-side association of DNA fibres. At an increase of ionic strength there is a tendency for transition from the second to the third type structures. The latter can be explained by transition from non-cooperative to cooperative binding of histone H1 to DNA. Regularities established for interaction between DNA and histone H1 can be useful for understanding the role of histone H1 in the higher order structure organization of chromatin.