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.