Organization of highly purified calf thymus DNA. I. Cleavage into subunits and release of phosphopeptides.

DNA (N-DNA) prepared under conditions eliminating the exposure of chromatin to cytoplasmic components exhibits some special properties not observed for DNA prepared by standard methods (S-DNA).N-DNA, having a sedimentation coefficient of 24.7 S and a firmly bound protein content of 0.7%, can be cleaved (in contrast to S-DNA) by treatment with chelating agents, into stable subunits having a mean molecular weight of about 500 000. This cleavage was shown to be an ordered process which involved no enzymatic or shear degradation. It was accompanied by the release of phosphopeptides. The analyses of these phosphopeptides revealed the presence of two main fractions. One contained phosphoserine and glycine (Mr about 1400), and the other contained phosphoserine, glycine, alanine, glutamic and aspartic acids (Mr about 900). The amount of released phosphopeptides could be correlated to the extent of cleavage.