Macromolecules influencing the tissue-specific restriction of DNA in chromatin from rat liver and thymus were studied. It was found that the specificity of RNA synthesis in vitro (as detected by DNA-RNA hybridization) can be changed from the pattern characteristic for rat liver to that of rat thymus and vice versa by exchanging the nonhistone proteins associated with the DNA in chromatin and reconstituting this fraction back to the DNA and histones by gradient dialysis. The origin of the histones did not contribute to the specificity of DNA restriction in isolated and reconstituted chromatin. Even the reconstitution of calf thymus histones to dehistonized rat liver chromatin did not alter the specificity of RNA normally synthesized by rat liver chromatin templates.
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