Marta Czupryn

HMG-like proteins of Physarum polycephalum: Association with the transcriptionally active chromatin

HMG‐like proteins were isolated from nuclei of the acellular slime mold Physarum polycephalum. These proteins represented 0.6 ± 0.2% of nuclear proteins and approx. 6% of DNA by weight. Polyacrylamide gel electrphoresis demonstrated the presence of 3 major proteins, designated HMG‐1P, HMG‐2P and HMG‐14/17P. None of these proteins comigrated with any of the calf thymus HMGs. Physarum HMG‐like proteins were found to be preferentially associated with transcriptionally active chromatin fraction, as well as being released from nuclei by light DNase I digestion. In contrast, the content of histone H1 was significantly lower in active than in inactive chromatin fraction.

Nonhistone proteins of the transcriptionally active chromatin fraction of Physarum polycephalum, associated with nucleosome linker DNA instead of histone H1

Physarum polycephalum Nonhistone protein Histone H1 Transcriptionally active chromatin

Chromatin reorganization during early differentiation of Physarum polycephalum

Abstract When microplasmodia of Physarum polycephalum are induced to differentiate by starvation, the rate of RNA synthesis measured in vitro with endogenous RNA polymerase B drops rapidly, decreasing after 12 h of starvation to 30% of the initial value. Transcriptional inactivation is accompanied by: (a) 60% decrease in the amount of the chromatin fraction solubilizable by light DNAase I digestion; (b) increase in the content of histone H1 and several nonhistone proteins in the DNAase-I-solubilized chromatin; and (c) formation of a nucleosome-like structure that is absent from this fraction during normal growth. No marked change in the structure and composition of the total chromatin on starvation is evident. These data suggest that the repression of transcription during differentiation in Physarum is correlated with structural reorganization of active chromatin regions.

An altered conformation of nucleosomal core particle in the active chromatin of physarum polycephalum

We show that nucleosomal particles from transcriptionally active chromatin (MgCl2-soluble fraction) of Physarum polycephalum have lower electrophoretic mobility than those of the bulk chromatin (MgCl2-insoluble fraction). Altered electrophoretic behaviour is observed even at the level of the nucleosomal core, and is shown not to result from its different DNA, RNA or protein content, nor to be a consequence of the process of DNA replication. Analysis of nucelosomes from both fractions on density gradient gels reveals that the slow migration of active nucleosomes is due to increased friction, i.e., more asymmetrical (unfolded) conformation of the nucleosomal core.