Chromatins of four embryonic chick tissues were digested with DNAase II and the MgCl2-soluble and insoluble chromatin fractions were isolated. The MgCl2-soluble fractions displayed a high protein/DNA ratio and enrichment of nascent RNA as compared to the MgCl2-insoluble fractions. The recovery of DNA and protein in the MgCl2-soluble chromatin of erythrocytes was much lower than that in the other cell types. The difference is likely to be associated with the low transcriptional activity of the erythrocyte chromatin. In polyacrylamide gels histones appeared to be the predominant protein constituent of the MgCl2-insoluble chromatins. Brain, skin and muscle displayed an apparently similar group of actively labelled nonhistones at 70 000 to 100 000 daltons. These nonhistones were not observed in erythrocytes. The MgCl2-soluble chromatins of erythrocytes, brain, skin and muscle had a prominent group of polypeptides at approx. 40 000 daltons. In all tissues except for erythrocytes the group of proteins was actively labelled. These polypeptides are suggested to be the major structural protein constituent of the MgCl2-soluble chromatin.
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