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Structures retaining many of the morphological features of nuclei may be released by gently lysing human cells in solutions containing non-ionic detergents and high concentrations of salt. These nucleoids contain superhelical DNA. Using a double-labelling procedure we have compared, at different salt concentrations, the amounts and types of protein associated with human nucleoides containings superhelical or relaxed DNA. We find that the slightly lysine-rich histones (H2A and H2B) but not the arginine-rich histones (H3 and H4) dissociate more slowly from nucleoids containing superhelical DNA than from those containing relaxed DNA. A protein of apparent molecular weight of 22000 also binds more tightly to superhelical DNA. We conclude that this protein and the slightly lysine-rich histones transmute free energy of supercoiling into binding energy when they bind to superhelical DNA.

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When HeLa cells are lysed in solutions containing a non-ionic detergent and 2 M-NaCl, structures are released that retain many of the morphological features of nuclei. These nucleoids contain all the nuclear RNA and DNA but few of the proteins characteristic of chromatin. Their DNA is supercoiled and so intact. Using a simple and rapid procedure we have reconstructed nucleohistone complexes from nucleoids and the ‘core’ histones without breaking the DNA. We have probed the integrity and structure of the reconstructed complexes using a non-destructive fluorometric approach, which provides a general method for detecting agents that bind to DNA and alter its supercoiling. The superhelical status of the DNA in the reconstructed complexes is indistinguishable from that found in control nucleoids containing core histones. Experiments with micrococcal nuclease confirm that the DNA in the reconstructed complexes is organized into nucleosome-like structures. These, however, are spaced 145 base-pairs apart and not 200 base-pairs apart as is found in native chromatin.

In the version of this article initially published, on p. 257, Steven Salzberg's name was misspelled as "Stephen. " The error has been corrected in the HTML and PDF versions of the article.