Transcription in acellular slime moulds

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.

Processing in the external transcribed spacer of ribosomal RNA from Physarum polycephalum

The rDNA of the myxomycete Physarum polycephalum is transcribed to give a 13.3 kb precursor of ribosomal RNA. At 1.7 kb downstream of the primary initiation site there is a processing site or a second initiation site. This site was studied by S1-mapping, DNA sequencing and electron microscopy. None of these methods could conclusively distinguish between the two formal possibilities. However, capping experiments indicate that rapid processing is taking place at this site rather than reinitiation. In addition, primary transcripts and processed molecules were assayed throughout the synchronous mitotic cycle. During all interphase stages newly initiated transcripts of rDNA and products of the first processing step are present in similar amounts, indicating control of initiation and not of maturation as being the main regulatory step for the accumulation of mature rRNAs. During the brief period of mitosis the level of newly initiated rRNA precursors is lowered.