Patterns of histone acetylation in the cell cycle of Physarum polycephalum

Labeling of histones in the naturally synchronous cell cycle of Physarum polycephalum with short pulses of tritiated acetate in vivo clearly showed three distinct patterns of histone acetate turnover. In G2 phase, turnover of acetate was observed only in histones H3 and H4, predominantly on the multiple acetylated forms. No acetate turnover was found in histones H2A and H2B compared with histones H3 and H4. In S phase, intense labeling was seen in all four core histones, in histones H3 and H4 predominantly in the low acetylated forms. In addition, cotranslational acetylation of the amino-terminal serines of histones H4 and H1 was observed during S phase. During mitosis, from condensation at prophase to decondensation after telophase, acetate turnover is almost zero. This suggests that within the mitotically condensed chromosomes all potential histone acetylation sites are masked. In G2 phase, when transcription is occurring, only histones H3 and H4 are available for acetate turnover, but in S phase, when both transcription and replication occur, all four histones are available for acetate turnover.