High diadenosine tetraphosphate (Ap4A) level at initiation of S phase in the naturally synchronous mitotic cycle of Physarum polycephalum.

Levels of the diadenosine tetraphosphate Ap4A are high during exponential growth of Physarum, decrease during encystment (spherulation) and increase again during excystment. Consistently, a rapid 8-30-fold increase in Ap4A level occurs at entry into S phase of the mitotic cycle and is maintained during the first half of genome replication. The elevated Ap4A level depends significantly on ongoing DNA replication and is completely sensitive to the protein synthesis inhibitor cycloheximide administered either before or after initiation of S phase.