Physarum polycephalum Histones

Physarumpolycephalum is an acellular slime mould whose histones are of general interest because they allow comparisons to be made between lower eukaryote and higher eukaryote histones and, perhaps more importantly, they provide a unique opportunity for the study of histone changes in the mitotic cycle, which is naturally synchronous in P. polycephalum plasmodia, and during differentiation. Studies of histone H 1 phosphorylation, in particular, have led to the proposal that this modification initiates chromosome condensation (Bradbury e f al., 1972, 1973; Matthews et al., 1976). Fractionation of P. polycephalum histones was first reported by Mohberg & Rusch (1969), who labelled the bands obtained by polyacrylamide-gel electrophoresis in reverse order of their mobilities: bands 1,2a, 2b, 3,4,5,6. Band 1 probably corresponds to histone H1 in higher eukaryotes because of its high mol.wt. (25000; Jockhusch & Walker, 1974), high lysine content (Mohberg & Rusch, 1969) and growth-associated phosphorylation (Bradbury et al., 1973). It is shown below that band 1 also behaves like histone H1 on gel filtration and ion-exchange columns. Bands 2a, 2b and 5 are minor components and on high-resolution gel electrophoresis bands 2a and 2b are resolved into five bands (results not shown). The mobilities of these minor bands do not correspond to any of the five major histone classes, HI , H2A, H2B, H3 or H4. Physarum band 6 co-migrates with calf thymus H4 (Mohberg & Rusch, 1969; Bradbury et al., 1973) and shows electrophoretic heterogeneity (Fig. 1). The scan of the stained gel shows two main bands that co-electrophorese with the two bands of calf histone H4 and several other minor bands. A short pulse-label with [14C]acetate labels the slower bands selectively, and the ratios [14C]acetate/[2H]lysine are proportional to 0, 1.0,2.2 and 2.8 for the four main bands, suggesting that the heterogeneity is due to zero to three acetate groups, as is found in higher eukaryotes. Changes in this pattern through the cell cycle have been studied (Robinson, 1976).