Activation of histone kinase in G2 phase of the cell cycle in Physarum Polycephalum

Abstract

Phosphorylation of histone Hl is one of the major chemical changes associated with growth in normal and cancer cells of eukaryotic organisms [I ,2] . The mycomycete Physarum polycephalum is an excellent model system for studying cell cycle events and it has been used to establish the time course of histone phosphorylation in the cell cycle [ 1 ] . In mid G2 phase nuclear histone kinase activity rises sharply, preceding an equally sharp rise in Hl histone phosphate content in prophase which may initiate chromosome condensation and hence mitosis [3] . Dephosphorylation of Hl occurs late in mitosis. These changes are regarded as an important part of the sequences of events which determine the progress of the nucleus through G2 phase [3-S]. Recently, studies with synchronised CHO cells have shown that the main changes in Hl phosphate content also occur in these mammalian cells [6]. The growth-associated histone kinase, kinase-HKG, has been extracted from Physarum nuclei and partly purified and characterised ([7] , unpublished data). A similar enzyme, that apparently crossreacts immunologically with the Physarum kinase-HKG ([8] , unpublished data), has been isolated from Ehrlich ascites cells and purified to homogeneity [9,10] . The Ehrlich ascites kinaseHKG phosphorylates Hl in vitro at sites including threonine-16, -136, -153 and serine-180 which are the sites phosphorylated in vivo during growth [ 1 l] . The magnitude of the increase in kinase-HKG activity in the Physarum growth cycle is 15-fold [3] . The ques-