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.

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Nucleosomal core particles containing different variants of histones H2A and H2B were isolated from actively growing sepals of lily buds (Lilium cundida, L.). The particles had decreased electrophoretic mobility relative to those from other sources. Comparison of the arrangement of histones along DNA in lily nucleosomes with previously obtained data for nucleosomes from yeast and animal cells, either active or repressed with regard to transcription and replication, has revealed no significant differences. The results suggest that the structure of nucleosomal core particles is highly conserved in all eukaryotes, whatever the functional state of the cell and the structure of histones H2A and H2B.

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