Phase-dependent polymerization and depolymerization of actin in Physarum polycephalum nuclei

G- and F-actin contents in physarum polycephalum nuclei isolated during the progress of cell cycle were estimated and compared both by the inhibiting activity of deoxyribonuclease I (DNase I) and by the fractionation on DEAE-Toyopearl column chromatography. The inhibition of DNase appeared maximal in late S phase or early G2 phase and decreased to a minimal value in late G2 phase or M phase, while total actin content in a nucleus, which was analyzed by SDS-polyacrylamide gel electrophoresis as well as by DNase inhibition assay in the presence of 0.75 M guanidine-HCl, did not show such phase-dependent dynamics through the cell cycle.