The longevity and reversibility of quiescence in  Schizosaccharomyces pombe  are 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 HIRA histone chaperone complex is required for the reversibility and longevity of nitrogen starvation-induced quiescence in  Schizosaccharomyces pombe . The HIRA protein, Hip1 is not required for entry into G0 or the induction of autophagy. Although  hip1 Δ cells retain metabolic activity in G0, they rapidly lose the ability to resume proliferation. After a short period in G0 (1 day),  hip1 Δ mutants can resume cell growth in response to the restoration of a nitrogen source but do not efficiently reenter the vegetative cell cycle. This correlates with a failure to induce the expression of MBF transcription factor-dependent genes that are critical for S phase. In addition,  hip1 Δ G0 cells rapidly progress to a senescent state in which they can no longer re-initiate growth following nitrogen source restoration. Analysis of a conditional  hip1  allele is consistent with these findings and indicates that HIRA is required for efficient exit from quiescence and prevents an irreversible cell cycle arrest.
Gal C, Cochrane GA, Morgan BA, Rallis C, Bähler J, Whitehall SK