Resolution of a spectrum of nucleoprotein species in sonicated chromatin

Sonicated rabbit-liver chromatin is fractionated by ion-exchange chromatography on ECTHAM-cellulose, a weakly cationic adsorbent. The chromatographic procedure provides a series of fractions having a spectrum of thermal denaturation profiles. The earliest-eluting fractions melt cooperatively at a significantly higher temperature than bulk chromatin, and totally lack any of the components of chromatin that melt at low temperatures. In contrast, the latest-eluting fractions are significantly enriched in their content of DNA sequences that melt at low temperatures. Although the protein to DNA mass ratio of all fractions is equal to that of unfractionated chromatin, polyacrylamide gel electrophoresis demonstrates that the relative amounts of individual proteins vary across the chromatographic peak. The most pronounced changes observed are the enrichment of a particular high molecular weight nonhistone protein in the later fractions, and a significant diminution of lysine-rich histone in the last fractions. The results, in conjunction with the demonstration by McConaughy, B. L. & McCarthy, B. J. (Biochemistry 11, 998-1003, 1972) that RNA transcribed in vivo in immature chick erythrocytes hybridizes to the DNA of only the portions of chromatin from these cells that melt at low temperatures, suggest that this fractionation may separate transcribable DNA sequences in chromatin from the repressed segments of a eukaryotic genome.