Control of histone gene expression in Physarum polycephalum. I. Protein synthesis during the cell cycle.

Synchronous cultures of Physarum polycephalum were pulsed with [3H]lysine hydrochloride in S and G2 phases of the cell cycle. Plasmodial extracts were separated into nuclear, ribosomal and acid-soluble post-ribosomal cytoplasmic fractions. Core histones could be detected by staining in the nuclear fractions of both S and G2 phases, but were not detected by staining in the cytoplasmic fractions. Newly synthesized histone was present in S-phase nuclei but not in S-phase cytoplasm. The specific activity of newly synthesized histone in G2-phase nuclei decreased by at least 95% compared to S phase and no newly synthesized histone was observed in G2-phase cytoplasmic fractions. Thus histone synthesis is restricted to S phase. There are no free pools of histone in the cytoplasm of Physarum in either S or G2 phases of the cell cycle.