Properties of Nuclease-resistant Fragments of Calf Thymus Chromatin

Nuclease-resistant chromatin fragments have been isolated on the basis of their insolubility in 0.15 m NaCl after limited digestion of calf thymus chromatin with micrococcal nuclease. Such fragments exhibit an unusual circular dichroism spectrum in the range from 260 to 300 nm, which is unlike that of B form DNA or undigested chromatin and most closely resembles that of C form DNA. Electric dichroism and sedimentation velocity measurements suggest that the DNA in these fragments is folded into a compact, perhaps superhelical, configuration. The average A258 nm: A230 nm ratio for six preparations was 1.27 ± 0.02, indicating a well defined composition of approximately 1.85 mg of protein per mg of DNA.