Histone proteins in vivo: cell-cycle-dependent physiological effects of exogenous linker histones incorporated into Physarum polycephalum.

We detail a method which allows biochemical quantities of histone proteins to be introduced into a living eukaryotic cell. This method involves absorption of purified proteins into macroplasmodia of Physarum polycephalum. Further, since Physarum macroplasmodia exist as syncitial culture with completely synchronous nuclei with respect to cell cycle events, proteins may be introduced at specific points during the eukaryotic cell cycle. We show that a linker histone is absorbed whole into these cells and are properly transported to the nuclei of the cell. Furthermore, we also show incorporation of linker histone H5 inhibits the transcriptional activities occurring during the G2 phase in Physarum. This method will make it possible to introduce histones modified with structural probes into chromatin naturally assembled in vivo.