Cell cycle dynamics regulate H3K27 and H3K9 histone modifications in Drosophila.

Cell cycle progression presents a fundamental challenge to epigenome integrity, particularly due to the need to reestablish post-translational histone modifications (PTMs) following DNA replication. Although proliferative and differentiating tissues exhibit markedly different cell cycle dynamics, how these differences shape the histone modification landscape in vivo remains largely unexplored. Here, we show that levels of H3K27ac, H3K27me3, and H3K9me3 are tightly linked to cell cycle dynamics in the Drosophila wing imaginal disc. We demonstrate that both physiological and pathological elongation of the cell cycle led to an accumulation of H3K9me3 and H3K27me3, whereas cell cycle acceleration reduces their levels. In contrast, H3K27ac exhibits the opposite pattern: levels decrease in arrested cells and increase with faster cycling. Genome-wide CUT&Tag analysis reveals that these changes predominantly affect genomic loci already modified in normally proliferating tissue. Importantly, the regulation of methylation levels at H3K9 and H3K27 is not solely mediated by the cell cycle machinery but reflects a metabolically guided process in which the rate of methylation is coupled to the rate of cell proliferation through metabolic activity, including signaling via the Insulin/PI3K/Akt pathway. Our study thus reveals key principles for understanding histone methylation in proliferating, senescent, and differentiating cells. In contrast, H3K27 acetylation is regulated through a distinct, cell cycle-coupled mechanism. We find that CBP/Nejire-mediated acetylation of H3K27 peaks during early and late S-phase and is reversed by HDAC1, as cells exit replication. Together, our findings establish a robust link between cell cycle progression and histone modification dynamics, highlighting the necessity of maintaining balanced PTM levels under varying proliferative states. These insights have broad implications for our understanding of development, aging, and tumor growth.