A chromosomal phosphoprotein is preferentially released by mild micrococcal-nuclease digestion

[32P]Pi was administered to rats (5mCi/rat) 2h before the isolation of liver nuclei. The isolated nuclei were subjected to mild micrococcal-nuclease digestion for 2.5, 5 and 10 min at 37 degrees C, and the mononucleosomal fraction was subsequently isolated by sucrose-density-gradient centrifugation. The specific radioactivity of 32P-labelled mononucleosomal fractions decreased with increased digestion times. A phosphorylated chromosomal protein, B2 (Mr 68000, pI6.5-8.2), was demonstrated immunologically in the mononucleosomal fraction by using an antibody specific to this electrophoretically purified phosphoprotein. The incorporation of 32P into this phosphoprotein, previously shown to be mainly through covalent linkage, was revealed by antibody precipitation followed by gel electrophoresis. The rate of release of acid-soluble nucleotides by micrococcal-nuclease digestion of liver nuclei from partially hepatectomized rats 16 h after operation was strikingly higher than that for sham-operated controls. After partial hepatectomy, an increase in 32P incorporation into phosphoprotein in the monomer fractions specifically precipitated by this antibody was also found. This suggests that the phosphorylated non-histone chromatin protein B2 is preferentially associated with the transcriptionally active chromatin.