Purification and characterization of the cytoplasmic histone acetyltransferase B of maize embryos

From a soluble cellular fraction of maize embryos we purified to apparent homogeneity a cytoplasmic histone acetyltransferase, which matches all criteria for a B‐type enzyme. Using 8 chromatographic steps, we achieved a 6700‐fold purification of an enzymatically active protein with a molecular weight of ∼ 90 kDa. Under denaturing conditions the protein split into 2 components which migrated at 45 and 50 kDa in SDS‐PAGE, suggesting that the native enzyme is a heterodimer. The purified enzyme was characterized in terms of physicochemical and kinetic properties, and substrate specificity. It was specific for histone H4, leading to acetylation of non‐acetylated H4 subspecies into the di‐acetylated state in vitro. Its activity was coincident with the intensity of DNA replication in meristematic cells during embryo germination. We established an electrophoretic system under non‐denaturing conditions for detection of enzyme activity within the gel matrix; in combination with second dimension SDS‐PAGE the procedure allowed the unambiguous identification of histone acetyltransferase, even in crude enzyme preparations.