[Stabilization by visible light of macromolecular chromatin complexes in the presence of ethidium bromide]

A method of induction of multiple DNA-protein and protein-protein cross-links in chromatin of isolated nuclei has been developed. The cross-linking is brought about by mutual effect of fluorochrome ethidium bromide (EtBr) and irradiation with blue light. Electron microscopic analysis of nuclei isolated in solutions with various concentrations of divalent cations, stained with EtBr and irradiated with blue light, has demonstrated that the method leads to a selective stabilization of macromolecular complexes of chromatin against various treatments causing decompactization of native chromatin. Besides that, the stabilization of nucleoli and clusters of interchromatin granules takes place. On the other hand, the same treatment does not stabilize elementary chromatin fibers (about 30 nm thick), and the transition to the nucleosomal fiber occurs after extraction of histone H1 with 0.6 M NaCl. Electrophoretic analysis of proteins from control and irradiated nuclei shows the basic role of non-histone proteins in the stabilization. The data are discussed based on the assumption on the availability of some "intermediate" levels of chromatin compaction between 30 nm chromatin fiber and mitotic chromosome.