5-Methylcytosine levels in nucleosome subpopulations differently involved in gene expression

Sucrose density gradient centrifugation in the presence or absence of Na-EDTA and at different ionic strengths allows one to obtain well-defined nucleosome subpopulations the DNA of which, examined by gas chromatography-mass spectrometry, is in all cases hypermethylated as compared to spacer regions, but to a different extent for the different subpopulations. The various nucleosomes differ also in their content of histones and of high-mobility-group proteins, as well as in the levels of RNA polymerase activity associated with them. Such data suggest that these nucleosome subpopulations originate from chromatin fractions differently involved in gene expression.