Temporal synthesis and intranuclear accumulation of the nuclear acidic proteins during periods of chromatin reactivation in Physarum polycephalum

In the lower eukaryote Physarum polycephalum, depending upon the existing cell state (i.e., actively growing plasmodia or metabolically quiescent cysts), there is in the complement of acidic chromatin proteins certain “proliferation” or “nonproliferation” associated proteins. Nonproliferative microplasmodia can be induced to undergo a 12 hr period of physiological and metabolic reorganization resulting in mitosis, DNA synthesis, and the reestablishment of active synchronous growth. During the 12 hr period of chromatin reactivation the specific acidic proteins associated with inactive chromatin and nonproliferative cell states decrease in intranuclear concentration in a continuous and linear fashion. The specific proteins associated with metabolically active chromatin and proliferative cell states are synthesized preferentially at different times during the 12 hr transition period. While several of the proliferation-associated proteins increase continuously in intranuclear concentration during the reactivation period others show maximum increases in their intranuclear concentration during the 2 hr period just preceding mitosis and DNA synthesis. The changes which develop in the acidic chromatin and nucleolar proteins during the period of chromatin reactivation occur independent of and prior to DNA synthesis and mitosis. Incorporation studies using ¹⁴C]-glutamic acid have provided additional evidence for periods of pooled protein and delayed intranuclear binding. The relative specific activities of individual protein bands determined through gel radiography show temporal differences independent of intranuclear protein concentration.It has been estimated that the proteins which are synthesized and accumulate in the nucleus during periods of chromatin reactivation are present in intranuclear concentrations between 80,000 and 1,000,000 copies per nucleus.