Changes in nuclear protein during the cell cycle in cultured mast cells separated by zonal centrifugation.

1. Exponentially grown mouse mast cells (cell line P815, strain Y) were separated by zonal centrifugation on a Ficoll gradient. Fractions were allocated to different phases of the cell cycle according to the specific radioactivity of their DNA. 2. Histones were extracted and their thiol content was analysed. The proportion of reduced thiol increased in S phase, decreasing subsequently. 3. The phosphate content of histone F1 and of the other histones reached a peak in early and later S phase respectively. The incorporation of (32)P into these fractions showed a corresponding increase. 4. The timing of histone synthesis was examined. Incorporation of (14)C-labelled amino acids into the histone fractions took place at the same times as phosphorylation. 5. Acid nuclear proteins differ from the histones in incorporating labelled amino acids and (32)P fairly constantly through the cell cycle.