Histone acetyltransferases during the cell cycle and differentiation of Physarum polycephalum

The dynamic state of histone acetylation is maintained by histone acetyltransferases (HATs) and deacetylases. Cellular fractionation of plasmodia of Physarum polycephalum and partial purification of subcellular fractions by chromatography revealed the existence of a cytoplasmic B-type and four nuclear A-type HATs. The cytoplasmic B-enzyme was highly specific for histone H4, causing di-acetylation of H4 in vitro. The nuclear enzymes (HAT-A1 to HAT-A4) accepted all core histones as substrates, but differed by the preference for certain histone species. Enzymes were analyzed during the naturally synchronous cell cycle of macroplasmodia. Each of the enzymes had its individual cell cycle activity pattern, indicating diverse functions in nuclear metabolism. When growing plasmodia were induced to undergo differentiation into dormant sclerotia, an additional enzyme (HAT-AS) appeared at a late stage of sclerotization which correlated with differentiation-specific histone synthesis and acetylation in the absence of DNA replication. When dormant sclerotia were induced to reenter the cell cycle, a further enzyme form (HAT-AG) appeared during a short time period prior to the first post-germination mitosis. This enzyme had a strong preference for H2B, correlating with the overproportional in vivo acetate incorporation in H2B. Both differentiation-associated HATs were undetectable in growing plasmodia. The results demonstrate that different functions of core histone acetylation are based on multiple enzyme forms that are independently regulated during the cell cycle. Transitions from one developmental stage into another are accompanied by specific enzyme forms. With respect to recent data in the literature it may be assumed that these HAT-forms are subunits of a HAT-complex whose composition changes during the cell cycle and differentiation.