Macromolecular and cytological changes in fission yeast G0 nuclei.

When starved of nitrogen, fission yeast Schizosaccharomyces pombe cells enter a quiescent "G0" state with smaller nuclei and transcriptional repression. The genomics of S. pombe G0 cells has been well studied, but much of its nuclear cell biology remains unknown. Here we use confocal microscopy, immunoblots, and electron cryotomography to investigate the cytological, biochemical, and ultrastructural differences between S. pombe proliferating, G1-arrested, and G0 cell nuclei, with an emphasis on the histone acetylation, RNA polymerase II fates, and macromolecular complex packing. Compared to proliferating cells, G0 cells have lower levels of histone acetylation, nuclear RNA polymerase II, and active transcription. The G0 nucleus has similar macromolecular crowding yet fewer chromatin-associated multi-megadalton globular complexes. Induced histone hyperacetylation during nitrogen starvation results in cells that have larger nuclei and therefore less compact chromatin. However, these histone-hyperacetylated cells remain transcriptionally repressed with similar nuclear crowding. Canonical nucleosomes - those that resemble the crystal structure - are rare in proliferating, G1-arrested, and G0 cells. Our study therefore shows that extreme changes in nucleus physiology are possible without extreme reorganisation at the macromolecular level.