Periodic fluctuations of nuclear high mobility group like proteins during the cell cycle of Physarum polycephalum.

High mobility group like (HMG-like) nuclear proteins were isolated from plasmodia of the lower eucaryote Physarum polycephalum and characterized by different types of polyacrylamide gel electrophoresis. The synthesis of these proteins was measured during the naturally synchronous cell cycle of Physarum. The four HMG-like proteins (AS1-4) exhibit a pronounced cell cycle dependent pattern of synthesis: AS1 and AS4 have a clear maximum of synthesis in mid S phase with a basal synthesis during the entire G2 period. In contrast, AS2 and AS3 have little synthesis in S phase but a broad maximum in mid G2 period. The four HMG-like proteins have a very low synthesis in early S phase and late G2 period. In addition, other non-histone proteins, which are coextracted with the HMG proteins, exhibit distinct periodic synthesis patterns. A novel non-histone protein, which is the most abundant protein species in 0.35 M NaCl extracts, was detected. It exhibits a high rate of synthesis around the time of mitosis. In general, the results indicate that, in contrast to the main cytoplasmic proteins, most nuclear proteins are phase-specific with respect to their synthesis in the cell cycle.