Self‐assembled pearling structure of long duplex DNA with histone H1

We report that complexes of giant DNA molecules with histone H1 proteins form a pearl necklace‐like structure when the complexes are prepared by natural dilution from a high‐salt solution (2 m NaCl) to a low‐salt solution (0.2 m and 50 m m NaCl). We performed real‐time observations on the conformational changes of individual T4 phage DNA (166 kb) molecules in bulk solution by fluorescence microscopy. To identify H1‐binding regions on individual DNA molecules, we also performed immunofluorescence microscopic observations on the DNA–H1 complex spread on a glass surface. It was found that histone H1 binds DNA in a highly co‐operative manner and is accompanied by local folding of the DNA. On the basis of the experimental observations and a theoretical simulation, we propose a self‐assembling mechanism for the pearling structure.