Histone H4 gene is transcribed in S phase but also late in G(2) phase in Physarum polycephalum

The myxomycete Physarum polycephalum contains two types of H4 histone genes. Southern blotting of restriction endonuclease fragments of P. polycephalum DNA and hybridization to a cloned probe labelled by nick-translation indicate that there are only one or two copies of each H4 gene per haploid genome. A cloned homologous genomic probe was used to study the cellular abundance of H4 mRNA during the cell cycle. We report that the H4 mRNA is not only transcribed in S phase as previously described for other organisms but that transcription of the H4 gene also occurs at the end of G(2) phase. Since no translation of the histone messenger was observed in G(2) phase this suggests that the histone mRNA synthesized in G(2) constitutes a pool of molecules in anticipation of the next S phase.