Repeating oligonucleosomal units. A new element of chromatin structure

Abstract

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.

Clusters of nonhistone chromosomal protein HMG1 molecules in intact chromatin

“…The fraction extracted with 3.5% HC104 has been shown to contain almost pure histone Hl polymer [4]. A minor protein component of this fraction earlier referred to as Ml [4] was electrophoretically similar to HMGl ( fig.2 in [4]).…”

“…Intact cell nuclei were isolated from calf thymus, treated with MMB, and the polymer fraction containing histone Hl and nonhistone protein Ml was obtained as in [4], except that treatment of the isolated protein with iodoacetamide was omitted.…”

“…For molecular weight calibration, the following standard proteins (Boehringer) were used: ovalbumin, serum albumin, and DNA-polymerase I from Escherichia coli. Partial splitting of protein oligomers was done as in [4].…”

“…Samples of total acid-soluble protein, histone Hl and nonhistone protein HMGl from calf thymus were prepared as in [4].…”

“…Their organization reflects the supranucleosomal structure of chromatin. The methyWmercaptobutyrimidate mediated polymer of histone Hl isolated and analysed earlier [4] belongs to the second group. It is represented by a series of oligomers consisting of e.g., 12,24 and 36 molecules of histone Hl .…”