Cellular quiescence is dependent upon the HIRA histone chaperone

Quiescence (G0) is a reversible non-dividing state that facilitates cellular survival in adverse conditions. Here we demonstrate that the function of the HIRA histone chaperone complex is required for viability during nitrogen-starvation induced quiescence in Schizosaccharomyces pombe. G0 cells lacking the HIRA protein, Hip1 exhibit elevated levels of antisense ncRNAs and an increase in unrepaired DNA double strand breaks. Nitrogen-starved hip1∆ cells retain metabolic activity but, in contrast to wild type, rapidly lose the ability to resume proliferation. After a short period in G0 (1 day), hip1∆ mutants are able to resume cell growth in response to the restoration of a nitrogen source, but do not efficiently induce Start-specific gene expression and re-enter the cell cycle. However, hip1∆ cells rapidly progress to an unresponsive state and by 4 days in G0, the majority no longer initiate growth following nitrogen source restoration. Analysis using a conditional hip1 allele is consistent with these findings and indicates that HIRA is required for efficient exit from quiescence and prevents a permanent cell cycle arrest. Examination of nitrogen starved (quiescent) and re-fed fission yeast cells (wildtype and hip1 mutant)