Abstract
Salt extraction of chromatin causes a progressive loss of histones from the nucleoprotein. 0.45 M NaCl extracts F 1 histones. Above this salt concentration, all other histones are extracted non-selectively. After 2.0 M NaCl extraction, which removes histones, a considerable amount of protein remains attached to the DNA. Electron microscopic examination confirms the progressive removal of protein by salt with the release of fibres with the dimensions of DNA and reveals that, after 2.0 M NaCl extraction, there are some residual nucleoprotein aggregates distributed among DNA fibres. Removal of F 1 histone causes little change in the template activity of the residual nucleoprotein but template activity of the salt-extracted chromatin rises as the F 2 and F 3 groups of histones are being removed. Estimation of free DNA-phosphate groups demonstrates that 47% of these are free in chromatin; this fraction increases progressively as histones are removed but after extraction with 2.0 M NaCl, about 15% of the phosphate groups in DNA are still not free to react with toluidine blue.
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