Repeating oligonucleosomal units. A new element of chromatin structure

Supranucleosomal chromatin structure has been analysed by the use of histone H1 polymers crosslinked in nuclei and extended chromatin with bifunctional reagents methyl-4-mercaptobutyrimidate (MMB) and dimethyl suberimidate dihydrochloride. Almost pure H1 homopolymers were obtained in milligram amounts and examined for the distribution in molecular weights. The H1 homopolymer molecules both from nuclei and chromatin have been found to be integer multiples of an elementary structure (called "clisone") consisting of 12 histone H1 molecules. This finding strongly suggests that nucleosomal chains of chromatin are not uniform but rather organized as repeating oligonucleosomal units each consisting of 12 nucleosomes. Correlation between oligonucleosomal structures in nuclei and chromatin implies that a linearized nucleosomal chain retains the information on chromatin superstructure. The relation of the disclosed 12-nucleosome units to superbeads (nucleomeres) and other structures is discussed.