A nuclear elongation factor of transcription from Physarum polycephalum in vitro.

Homogenates of Physarum plasmodia contain a factor which stimulates UMP incorporation on native DNA by solubilized homologous RNA polymerases in vitro. The factor is a heat-sensitive protein and has been located in nuclei. It does not alter the template activity of DNA nor the initiation frequency of transcription. The factor interacts with free or bound RNA polymerase molecules (only 37 degrees C and at low ionic strength) and yields larger transcripts in vitro. The level of the factor in vitro fluctuates: it is gradually reduced during spherulation and reaches its maximum in mid S phase of the cell cycle of Physarum.