Electron-microscopic study of the structural changes in kinetoplast DNA induced by histone H1

Kinetoplast DNA, an association of DNA minicircles with attached maxicircles, exists in the cell in a compact state owing to bound basic histone-like proteins. The structural changes in isolated kpDNA caused by interaction with histone H1 were assessed by spreading the kpDNA on the surface of a hypophase with different H1 concentrations (protein-free spreading) and examining the resulting structures in an electron microscope, considering both integrated kpDNA networks and accompanying single minicircles. At 5-10 mu g/ml H1 in the hypophase, the networks had a typical well-spread recognizable structure of diameter equal to that of controls spread with cytochrome c. However, the constituent DNA fibers appeared thicker, consisting of several dsDNA filaments side by side. Single DNA minicircles were also compacted in the presence of H1 (linear compaction coefficient 3.7). Analysis of the structures suggested that in this case compaction is also due to side-by-side association of dsDNA segments, as shown earlier for ''triple rings'' formed by circular supercoiled DNA exposed to trivaline.