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By treatment with tRNA in the presence of 1 mM MgC12, a chromatin preparation was obtained containing all five major histone fractions but lacking a considerable portion of non-histone proteins. This chromatin preparation as well as chromatin extracted with 0.6 M NaCl (depleted of H1 histone and some non-histone proteins) were characterized in respect of solubility and chromatin DNA accessibility.Both samples possessed practically the same solubility in the presence of 0.15 M NaCl and 1 mM MgC12. The solubility of tRNA-treated chromatin in 5 and 10 mM MgClz was higher than that of salt-extracted chromation.The accessibility of the DNA of these chromatin preparations was tested with DNA-dependent RNA polymerase of Escherichia coli as a probe, using procedure that permits measurement of binding site frequency. Both tRNA-treated and salt-extracted chromatin contained as many as 33 '%, and untreated chromatin as few as 4 ;/; of the number of binding sites found on protein-free DNA.These results demonstrate that at least in part the non-histone proteins are responsible for saltinduced insolubility and low DNA accessibility of chromatin, thus revealing the importance of nonhistone proteins in the maintainance of an overall Chromatin structure.The precise role played by histones and nonhistone protein in the structure and function of chromatin is still not clear. Morphological and biochemical evidence strongly supports the concept that the four small histones form the structural repeating unit of chromatin : the nucleosome (for review see [l]). But the function of H1 histone in chromatin is evidently distinct from that of the other histones. By analyzing selectively dehystonized chromatin it has been suggested that H1 histone is responsible for chromatin contraction and precipitation in 0.1 5 M NaCl [2,3] as well as for the cross-linking of chromatin fibers that results in massive condensation such as heterochromatization [4-61.However, the extraction of H1 histone is usually accompanied by the removal of considerable portion of non-histone proteins [7-101. Since the techniques used to remove H1 histone also result in a loss of nonhistone proteins, it is difficult to ascribe changes in properties of H1 -depleted chromatin purely to the loss of histone H1. It is generally believed that non-histone (EC 2.7.7.6).proteins play an important role in regulating transcription of DNA [l]. Moreover, recently Ide et al. [I 11 have shown that non-histone proteins participated in supercoiled folding of chromatin DNA. Therefore, it would be of interest to examine the influence of selective removal of some non-histone proteins on the properties of chromatin.In this paper we describe the preparation of chromatin in which only part of the non-histone proteins was removed without the loss of five major histone fractions. The structural properties of this chromatin preparation as well as of 0.6 M NaCI-extracted chromatin (depleted of H1 histone and some non-histone proteins) are characterized in respect of solubility and chromatin DNA ...

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