[Electron microscope study of changes in the chromatin structure in different functional states of nuclei of a ciliate Bursaria truncatella and a myxomycete Physarum polycephalum].

Chromatin structural organization was studied by means of electron microscopy in the macronuclei of ciliate Bursaria truncatella at various stages of the life cycle (at different time intervals after cell division, in resting cysts and at excysting) and in the nuclei of myxomycete Physarum polycephalum during the mitotic cycle. Inactive chromatin was shown to be organized in compact clumps 100-300 nm in diameter linked with each other, their loop organization being convincingly demonstrated. Upon activation chromatin decompacts and is represented by nucleosomal fibres with a lot of replicationally and transcriptionally active regions. Both the reptication and transcription processes in physarum nuclei and transcription processes in bursaria can occur on the loops of chromatin fibres emerging from the decompacting clump. The data obtained evidence in favour of a structural-functional correspondence between the chromatin clumps of physarum and bursaria and the chromomeres in chromosomes of higher eucaryotes. Based on the data received it is concluded that the chromatin clump represents a dynamic structure unit able to decompact forming the loop-shaped chromatin fibres. Such a structural organization provides the spacial distribution of DNA in the nuclei and the possibility of selective functioning of definite regions of the genome.