Change in chromatin organization related to in vivo transcriptional activity and histone synthesis independent of DNA replication during differentiation (germination) of Physarum spherules.

During the germination of Physarum spherules, increases have been observed, at the same moment, in the level of in vivo transcriptional activity as measured by [3H] uridine incorporation, and the accessibility of DNA for ethidium bromide staining as shown by flow cytometric measurements. We suppose that the changes observed in these two processes are due to a difference in chromatin organization between the first and the second period of the premitotic germination stage. In the second period, the four nucleosome core histories are synthesized in the absence of DNA replication and may correspond to a replacement of spherulation histone variants by plasmodial histone types in nucleosomes. The synthesis of historic H4 clearly distinguishes the second period of the premitotic germination stage from a growing plasmodium G2 phase, though nuclei exhibit a G2 phase DNA content. The same pattern of histone synthesis has been found during the cell cycle following the first mitosis after germination and the growing plasmodium cell cycle, with a synthesis of two histories H2B and H2A and the high mobility group (HMG)-like protein AS3 during the G2 phase, i.e. in the absence of DNA synthesis.