An essential role for the Ino80 chromatin remodeling complex in regulation of gene expression during cellular quiescence

Cellular quiescence is an important physiological state both in unicellular and multicellular eukaryotes. Quiescent cells are halted for proliferation and stop the cell cycle at the G₀ stage. Using fission yeast as a model organism, we have previously found that several subunits of a conserved chromatin remodeling complex, Ino80C (INOsitol requiring nucleosome remodeling factor), are required for survival in quiescence. Here, we demonstrate that Ino80C has a key function in the regulation of gene expression in G₀ cells. We show that null mutants for two Ino80C subunits, Iec1 and Ies2, a putative subunit Arp42, a null mutant for the histone variant H2A.Z, and a null mutant for the Inositol kinase Asp1 have very similar phenotypes in quiescence. These mutants show reduced transcription genome-wide and specifically fail to activate 149 quiescence genes, of which many are localized to the subtelomeric regions. Using spike in normalized ChIP-seq experiments, we show that there is a global reduction of H2A.Z levels in quiescent wild-type cells but not in iec1∆ cells and that a subtelomeric chromosome boundary element is strongly affected by Ino80C. Based on these observations, we propose a model in which Ino80C is evicting H2A.Z from chromatin in quiescent cells, thereby inactivating the subtelomeric boundary element, leading to a reorganization of the chromosome structure and activation of genes required to survive in quiescence.