Biosynthesis and posttranslational acetylation of histones during spherulation of Physarum polycephalum

Plasmodia of Physarum polycephalum can be induced to differentiate into dormant spherules: DNA-, RNA- and protein-synthesis cease during this process. Analysis of the histone H4 acetylation during spherulation revealed no significant changes of the relative acetate content and percentage of acetylated H4 subspecies. This result does not support a close correlation of histone acetylation and transcriptional activity. Posttranslational incorporation of 3H-acetate into core histones decreased rapidly after start of spherulation. However, acetate incorporation increased significantly at a late stage of spherulation (30 h). To elucidate the role of this elevated acetate incorporation we followed histone synthesis during spherulation. Histone synthesis decreased upon induction of differentiation and stopped after 12 h. After 38 h of spherulation histone synthesis again occurred in the absence of DNA synthesis. The peak of acetate incorporation into core histones clearly preceded this late histone synthesis, indicating acetylation of preexisting histones. We suggest, that this acetate incorporation is part of the mechanism, by which preexisting histones are replaced by newly synthesized histones. Pulse treatment with actinomycin D or cycloheximide during spherulation suggested, that the observed histone synthesis is essential for the germination of spherules. Obviously, new histones have to be synthesized for the coordinate course of the differentiation program.