Histone modifications direct chromatin-templated events in the genome and regulate access to DNA sequence information. There are multiple types of modifications, and a common feature is their dynamic nature. An essential step for understanding their regulation, therefore, lies in characterizing the enzymes responsible for adding and removing histone modifications. Starting with a dosagesuppressor screen in Saccharomyces cerevisiae, we have discovered a functional interaction between the acetyltransferase Gcn5 and the protein phosphatase 2A (PP2A) complex, two factors that regulate post-translational modifications. We find that RTS1, one of two genes encoding PP2A regulatory subunits, is a robust and specific high-copy suppressor of temperature sensitivity of gcn5D and a subset of other gcn5D phenotypes. Conversely, loss of both PP2A Rts1 and Gcn5 function in the SAGA and SLIK/SALSA complexes is lethal. RTS1 does not restore global transcriptional defects in gcn5D; however, histone gene expression is restored, suggesting that the mechanism of RTS1 rescue includes restoration of specific cell cycle transcripts. Pointing to new mechanisms of acetylation-phosphorylation cross-talk, RTS1 high-copy rescue of gcn5D growth requires two residues of H2B that are phosphorylated in human cells. These data highlight the potential significance of dynamic phosphorylation and dephosphorylation of these deeply conserved histone residues for cell viability. KEYWORDS chromatin; transcription; phosphorylation; acetylation (IOC2, PAB1, RHO2, MED6, ZDS1, PP2A B56 ) A T the foundation of nuclear DNA organization in eukaryotes is the dynamic formation, movement, and modification of nucleosomes. Acetylation and phosphorylation are two histone post-translational modifications (PTMs) catalyzed by histone acetyltransferases (HATs) and kinases and reversed by histone deacetylases (HDACs) and phosphatases that alter nucleosome structure and function. Tightly regulated acetylation and phosphorylation of specific histone residues have deeply conserved functions in eukaryotes that are critical for transcriptional regulation, replication, repair, and segregation of eukaryotic genomes (Banerjee and Chakravarti 2011;Rossetto et al. 2012;Tessarz and Kouzarides 2014).One well-conserved HAT is Gcn5, which specifically targets histones H3 and H2B as part of multiple complexes (Grant et al. 1997;Eberharter et al. 1999;Grant et al. 1999;Howe et al. 2001;Sterner et al. 2002;Pray-Grant et al. 2005). Gcn5 is a key regulator of eukaryotic gene expression and acetylates H3 at promoters of active genes (Pokholok et al. 2005;Nagy and Tora 2007;Rosaleny et al. 2007). Its role as a transcriptional activator is so fundamental that Gcn5 is essential in most eukaryotes studied. An exception of note is budding yeast, where deletion of GCN5 is tolerated, but does cause a spectrum of phenotypes, including defects in gene activation, particularly for stress-regulated genes, and altered cell cycle progression (Howe et al. 2001;Huisinga and Pugh 2004;Vernarecci ...