A refined map for the linear arrangement of histones along DNA in nucleosomal core particles has been determined by DNA-protein crosslinking. On one strand of 145-bp core DNA, histones are aligned in the following order: (5') H2B25,35-H455,65-H375,85,95/H488-H2B105,11 5-H2A118-H3135,145/H2A145 (3') (the subscripts give approximate distance in nucleotides of the main histone contacts from the 5'-end). Hence, the histone tetramer (H3,H4)2 and two dimers (H2A-H2B) are arranged on double-stranded core DNA in a symmetrical and rather autonomous way: H2A/H3-(H2A-H2B)-(H3,H4)2-(H2B-H2A)-H3/H2A. The primary organization was found to be very similar in core particles isolated from repressed nuclei of sea urchin sperm and chicken erythrocytes, from active in replication and transcription nuclei of Drosophila embryos and yeast and from somatic cells of lily. These data show that (i) the core structure is highly conserved in evolution and (ii) the overall inactivation of chromatin does not affect the arrangement of histones along DNA and thus does not seem to be regulated on this level of the core structure.

No abstract

No abstract

The minor chromatin fraction was isolated from the sonicated calf thymus nuclei on the basis of its differential solubility in the "quasiphysiological" salt medium (0.1 M KCl-0.05 M NaCl-l mM MgCl2-1 mM CaCl2). Histone Hl is almost completely absent from this fraction. DNA isolated from this fraction occurs in three discrete low mol. wt. fragments. The fraction of chromatin which lacks histone Hl can also be obtained by two other methods. On of them consists in salt precipitation of the chromatin gel and its subsequent sonication. The second method includes precipitation of the sonicated chromatin gel by salts. In the first case the properties of the chromatin fraction which remains in the supernatant after centrifugation closely resemble those of the original salt-soluble nuclear fraction. The second method yields supernatant fraction also lacking histone Hl but containing heterogeneous DNA. Comparisons were also made of the sonically-solubilized nuclear fractions obtained in the complete salt medium and its mono and divalent cationic constituents.

No abstract