Usage of the H3 variants during the S-phase of the cell cycle in Physarum polycephalum

DNA replication occurring in S-phase is critical for the maintenance of the cell fate from one generation to the next, and requires the duplication of epigenetic information. The integrity of the epigenome is, in part, insured by the recycling of parental histones and de novo deposition of newly synthesized histones. While the histone variants have revealed important functions in epigenetic regulations, the deposition in chromatin during S-phase of newly synthesized histone variants remains unclear. The identification of histone variants of H3 and unique features of Physarum polycephalum provides a powerful system for investigating de novo deposition of newly synthesized histones by tracking the incorporation of exogenous histones within cells. The analyses revealed that the rate of deposition of H3.1 and H3.3 is anticorrelated as S-phase progresses, H3.3 is predominately produced and utilized in early S and dropped throughout S-phase, while H3.1 behaved in the opposite way. Disturbing the expression of H3 variants by siRNAs revealed mutual compensation of histone transcripts. Interestingly, the incorporation of pre-formed constrained histone complexes showed that tetramers of H3/H4 are more efficiently utilized by the cell than dimers. These results support the model whereby the histone variant distribution is established upon replication and new histone deposition.